if you could alter the contents of the hourglasses,
fill the 7 minute hourglass, then with its contents,
fill the 4 minute hourglass, now the former 7-minute hourglass has
is now a 3 minute hourglass, flip 3 times.

Just flip both leaving 3min left in the ⁷ then immediately flip the 4 and as soon as the 3min for the ⁷ ends start the souffle (there should be 1 min left in the ⁴). Then flip the ⁴ twice and you have 9 min.

another explanation (ratios are top/bottom):
t+0: flip both (7/0) (4/0)
t+4 (0/4): flip 4min (3/4) (4/0)
t+7 (0/7): flip 7min (7/0) (1/3)
t+8 (0/4): flip 7min (1/6) (0/4)
t+9 (0/7): remove souffle

sorry my mistake :(
graphical explanation ;)

\7/\4/ \3/\0/ \3/\4/ \0/\1/ \7/\1/ \6/\0/ \1/\0/ \0/\0/

 *  * > *  * > *  * > *  * > *  * > *  * > *  * > *  *

/0\/0\ /4\/4\ /4\/0\ /7\/3\ /0\/3\ /1\/4\ /6\/4\ /7\/4\

pass  4      0      3      0      1      0      1

sum   4      4      7      7      8      8      9

graphical explanation ;)

\7/\4/ \3/\0/ \3/\4/ \0/\1/ \7/\1/ \6/\0/ \1/\0/ \0/\0/

 *  * > *  * > *  * > *  * > *  * > *  * > *  * > *  *

/0\/0\ /4\/4\ /4\/0\ /7\/3\ /0\/3\ /1\/4\ /6\/4\ /7\/4\

pass  4      0      3      3      1      1      1

sum   4      4      7      7      8      8      9

There is a mistake in your explanation. i updated it (marked with >…<):

Turn at the same time both hourglasses. When the 4 min hourglass finish, turn it again. When the 7 min. hourglass finish, turn it. >At this time 7 min have passed and in the 4 min. hour glass is 1 min. left. When the 4 min. hourglass finish,< at this time 8 min. have pass and only one minute is needed >and this 1 min. is in the 7 min. hourglass, so < turn again the 7 min. hourglass. The sand that have fall is going to last one min. and you have count 9 min. in total.