There's only 1 purely legal place to go on oahu, the 4x4 park on sand island.
If you really just want to get off road and tear up the scenery, yea east side you'll need 4x4. Leeward not really. Some of peacock flats, but if you're taking an IFS truck up that, you'll probably be avoiding the tough stuff anyway. Ride in a friend's jeep, it'll be more fun.
I have 4x4 for "just in case" and I'm a bit of a "survivalist" type thing and am moving to mainland soon-ish. Comes down to, why not get the 4x4. But can you use it on oahu? Not really. Can you even come up with occasion to use it? Not unless you work construction/farming have a lot of land on windward side or have like family access to pig hunting trails.
I did get my 2wd truck stuck on my property up on the north shore once. But was a VERY particular circumstance that would never happen again.
You shouldn't be driving on the beach on oahu, so sand is out too.
So yea. Why not buy the 4x4 just in case. But if you're already stretching the budget. No.
Its not much of an "off roader" anyway unless you SAS the truck. IFS has like.. 6 inches of travel in the front. Rear isn't great either .
Mokuleia will beat the crap out of that suspension. No fun at all and its not designed for it. Get a dirt bike and head out to kahuku or down to sand island to get "tearing shit up" out of you.