I am going to order a cheap 4 channel esky flight sim controller and then use it with Realflight g3.5 or 4 or FSOne. Will this work?

I would say no.
My realflight G2 will only work with the included controller, I tried other controllers but they wouldn't work without the original hooked up.
I think it is made to work that way to prevent software piracy.

Are you sure it wont? Because my buddy will loan me one of his sims, but he cant loan me a controller.

Definitly not with G3.5 or 4. You have to have the interlink controller to use as an interface if you want to use an actual radio. As far as FS One goes, I don't know. Realflight is the only sim I have ever messed with.

there is an interlink emu out there.....;-)

Ahh. I found that! Its called dongle if I remember right!