I looked over the iPaq hx2790's specs and didn't see a mention of a capacity limit to SD and/or CF cards. The maximum size of SD cards that HP offers is 1GB, but that's not an official limit.
Now, if there is a 1GB technical limitation to the hx2790 then one of two things would happen if you attempt to use a 4GB card: It could either not be usable at all, or, more likely, only be recognized as a 1GB card, leaving the other 3GB unusable to the PDA. My suggestion would be to contact HP customer or technical support (click here) to find out for sure. They have released a couple ROM and firmware updates over the past year so it's possible that one of the updates has removed the limitation, if it did exist.
Hope this helps,
I am oing to purchase the HP ipaq hx 2790 but i have 1 issue that I need to sort out. The specification on the hp site states that the ipaq is only certified up to 1gb of sd/flash storage does that meana i cannot buy a 4gb sd card and use it. If i can does it cause any problems???