Was Brazil's 6-minute, 4-goal collapse the single biggest collapse in sports history considering the context that it happened in?

Too early but probably not.  It was bad no question and melting down that way at home makes it worse.  But it isn't even the most shocking poor performance in World Cup history.   Teams that go up 3-0 in 7 game series and then lose come to mind as greater collapses than this.

I'm inclined to say yes. The closest I can think of is probably Arsenal's capitulation against Liverpool earlier this year, and even that wasn't this bad. I'm curious to hear some comparable suggestions, but falling apart that spectacularly is difficult to match.

It has happened before. In 1970 England were 2-0 up against Germany in the semi-final with 15 minutes left when Sir Alf Ramsey made some tactical substitutions to rest players. England went on to lose 3-2. Germany has rebuilt the side after a similar humiliation losing 1-5 at home to 1966 World Cup winners England. The Germans created a 10 year plan to make their team more like England and worse still turned Sven Goran Erikkson into a national hero. Hull City suffered a collapse in form after manager Phil Brown forced the players to have their half time talk on the pitch. The team didn't win again that season. They were in contention for a top 4 finish at the time. To put the Brazil defeat in perspective: the team wasn't that good and had lost key players.

Most of the answers here are pertaining to football. But since the question asks about all sports, i would like to add an example from cricket. (those who don't follow cricket pardon me!) World cup 2011 India v south africa: Tendulkar and Sehwag gave India a rollicking start, and maintained a run rate of 6.5+ throughout. At 269/1 in 39 overs, we fanatic indian fans were looking for a 400+ score. What followed was carnage. India lost 9 wickets for a mere 29 runs to be dismissed for 296. Perhaps the batsmen too wanted a 400 score and so went for broke and ended up broken. It still was a close match though, but India lost. http://www.espncricinfo.com/icc_cricket_worldcup2011/engine/current/match/433586.html Of course, the IPL has such drama frequently, but it is so dramatic that it looks staged. So i am not including any of those matches here.

I would say that Devon Loch comes very close to Brazil and maybe even beats them. Devon Loch was a horse owned by the Queen Mother which was entered into the Grand National, by far the biggest horse race in the UK. Devon Loch was 5 lengths in front with all of the fences jumped and just 100 feet (30 metres) left to run.  For no apparent reason the horse jumped in the air, lay on its stomach and simply refused to move any further. The horse was apparently unhurt and perfectly well and 'doing a Devon Loch' has entered British and Irish English as a metaphor for grabbing an unlikely defeat when losing is harder than winning.

I'm afraid to say that there have been bigger and more humiliating defeats in football. 1. Liverpool vs AC Milan, Champions League Final 2005. Liverpool were 3 - 0 down at half time. Come the second half, they score 3 goals in a span of just 6 minutes. The match goes to extra time and liverpool win on penalties. 2. Liverpool vs Arsenal, Premier League 2014 Although the scoreline was not as intimidating as the Germany Brazil one but this match makes it to one of the most brutal attacks on a top in sport history. They scored 4 goals in a matter of 18 minutes and that's it. The Game is over. That was embarrassing for arsenal.

Not even close. How about this - in the 2013 America's Cup Team New Zealand had to win 9 races to claim the cup.   They won 8 and were only one win away from reclaiming the cup.  The make matters worse Team Oracle (their opponent) had been docked two race points prior to the cup starting.  So in order to win - Oracle would have to win 11 races and NZ only win 9. Read Team NZ's results and (if you're a Kiwi) weep.... Race 1 - Beat Oracle Team USA by 36 seconds (1-0) Race 2 - Beat Oracle Team USA by 52 seconds (2-0) Race 3 - Beat Oracle Team USA by 28 seconds (3-0) Race 4 - Lost to Oracle Team USA by 8 seconds (3-0) Race 5 - Beat Oracle Team USA by 1 minute 5 seconds (4-0) Race 6 - Beat Oracle Team USA by 47 seconds (5-0) Race 7 - Beat Oracle Team USA by 1 minute 6 seconds (6-0) Race 8 - Lost to Oracle Team USA by 52 seconds (6-0) Race 9 - Lost to Oracle Team USA by 47 seconds (6-1) Race 10 - Beat Oracle Team USA by 16 seconds (7-1) Race 11 - Beat Oracle Team USA by 15 seconds (8-1) Race 12 - Lost to Oracle Team USA by 31 seconds (8-2) Race 13 - Lost to Oracle Team USA by 1 minute 24 seconds (8-3) Race 14 - Lost to Oracle Team USA by 23 seconds (8-4) Race 15 - Lost to Oracle Team USA by 37 seconds (8-5) Race 16 - Lost to Oracle Team USA by 33 seconds (8-6) Race 17 - Lost to Oracle Team USA by 27 seconds (8-7) Race 18 - Lost to Oracle Team USA by 54 seconds (8-8) Race 19 - Lost to Oracle Team USA by 44 seconds (8-9) As you can see - after 11 races NZ was up 8 - 1.  One more race would do it.  But no - they then proceeded to lose the next 8 races in a row and therefore lose the cup. In this part of the world we don't call that a collapse. We call it a choke.

Germany 16 : 0 Russia Germany's 7:1 victory over Brazil pales in comparison to a game more than 100 years ago. During the 1912 Olympics in Stockholm, Germany and Russia met in a "pity round" of teams who did not make it into the regular tournament's semi-finals. The Germans surely found some comfort in beating Russia 16-0, which is the country's largest ever footballing victory.

Not at all... In the 2000 Copa Mercosul, Vasco da Gama and Palmeiras were playing a grudge match to undo a tie (the first game was 2-0 to Vasco da Gama and the second one was 1-0 to Palmeiras). This was the final match, of a 'South American Championship' and that's what happened: 36' - Vasco da Gama 0 x 1 Palmeiras 37' - Vasco da Gama 0 x 2 Palmeiras 45' - Vasco da Gama 0 x 3 Palmeiras Vasco da Gama went into the half-time almost in despair. Nobody believed that 10 minutes could put such a burden on a team. But when the second half began, something happened: 59' - Vasco da Gama 1 x 3 Palmeiras 68' - Vasco da Gama 2 x 3 Palmeiras 86' - Vasco da Gama 3 x 3 Palmeiras Nobody could believe. Until... 90' + 3' - Vasco da Gama 4 x 3 Palmeiras Vasco da Gama supporters went from hell to heaven in 45 minutes. Palmeiras supporters went from 'South American Champions' to 'Runner-ups'. Feelings were reversed and this is, until today, one of the most impressive final matches of Brazilian football's history.