Chopiabin, you seem to have left out the simple glissando-ed etude, #11 Op. 10. Next in line is #1 Op. 25, then you got:
#7 Op. 25
some say #2 Op. 25 should be next,
#3 Op. 10,
#5 Op. 25
and it's all uphill from there, really...
Technically hardest 4 etudes-
#6 Op. 25 - insane thirds; I just dont get it
#11 Op. 25 - stamina, speed mixed with seemingly random notes
#2 Op. 10 - chromatic NOT using first 2 fingers

#7 Op. 10 - alternating 3rds and sixths, just crazily
Check them all out- it's well worth it. I'd say that #4 is very hard, but actually not as much so as the above mentioned 4, because 4 is simply speed/timing and basic coordination between the hands to play rhythmically. It comes more with practice, as the others require learning a whole new tecnique (they put the 'etude' in 'etude').