Is TF4 really that successful? I don't think so, based on the current stats.

Comparing the opening week stats in North America to the previous movies...

TF4 (these are noted as estimates, but shouldn't be too far off the actuals)
F 27th June - 41.6 million (a lot less than TF2's opening day)
S 28th June - 32.2 million
S 29th June - 26.2 million (almost half of the first day)
(total for first week - 100 million... a lot less than the previous three)
The takings for the next five days will drop off (being weekdays), and second weekend takings are always a half to a quarter of first weekend takings. I don't see this movie making as much as the others in America... but will make up the difference in non-American markets.

TF3 (opened on Wednesday - the numbers held right through to the Sunday, which isn't happening with the Fourth movie)
W 29th June - 37.7 million
T 30th June - 21.5 million
F 1st July - 33.0 million
S 2nd July - 34.5 million
S 3rd July - 30.4 million (total for first week - 162.6 million)

TF2 (also opened Wednesday - after a huge first day, the numbers settled and stayed mostly steady until the Sunday)
W 24th June - 62.0 million
T 25th June - 29.1 million
F 26th June - 36.7 million
S 27th June - 40.2 million
S 28th June - 32.0 million (total for first week - 200.1 million)

TF1 (opened Tuesday)
T 3rd July - 27.9 million
W 4th July - 29.1 million
T 5th July - 19.2 million
F 6th July - 22.7 million
S 7th July - 25.7 million
S 8th July - 22.1 million (total for first week - 155.4 million)
(again, numbers held through to the end of the first weekend, something TF4 looks to have failed to achieve)

Based on audience comments and atmosphere in the sessions I was at, I think word of mouth discouraging others and a lack of repeat screenings by those who did see it, could be the reason for the much more rapid drop-off of interest in TF4 than the other three.