Where are you getting 25%? With four outs preflop to river, I get:
1-[ 44/48 * 43/47 * 42/46 * 41/45 * 40/44 ] = .3657
Also, and I don't know why it is different than my calculation:
http://twodimes.net/h/?z=1139012
pokenum -mc 500000 -o as ad ac ah - ts td 3c 3h
Omaha Hi: 500000 sampled boards
cards win %win lose %lose tie %tie EV
As Ac Ad Ah 304599 60.92 195401 39.08 0 0.00 .609
Ts 3c Td 3h 195401 39.08 304599 60.92 0 0.00 .391