would it be over board to put a sup tigre 2500 in a 4 -120?

No.

THIS is overboard.

http://www.rcuniverse.com/forum/m_3566766/tm.htm

It floats in for a landing. I haven't weighed the plane, but I think it's around 12 or 13 pounds. Even here in Colorado at darn near 5000' elevation, it just floats and floats and floats and floats. It will eat up the whole runway if I don't setup my approach about 200' off the end of the runway.

Engine was brand new. Still less than a gallon of fuel through it as of today. It's turning a Zinger 17-8 at 6200RPM and getting better every tank. I imagine it will take 3 or 4 gallons to really get broke in.

It's very relaxing to fly. This is my "Sunday Flyer" for those days when I don't want to scare myself with my usual 50cc gasser aerobatic planes. It's really a nice easy plane to fly. Easy to land. Easy to take off. Easy on the wallet with the Ryobi. It sucks down a whopping 12oz of fuel in about 40 minutes. I calculated it will take me somewhere in the neighborhood of 100 flights to get 5 gallons through the motor. Maybe then it will be broke in.

If you have the ST 2500--then do it. If you don't have an engine yet, then put a small gasser in it. You can even use some of the really small weedies like the Featherlite 18cc and 21cc engines with a glow plug and glow fuel. Or, you can do what I like to call a hybrid where you mix 2 gallons of glow to 1 gallon of gas and still use a glow plug. No ignition, no magneto, no coil. Just prime it--put the glow igniter on it and flip it.

I prefer gas though. I'm trying to get away from glow as much as possible. You can do a Ryobi 31 on magneto ignition for about $140. Put a C&H on it, like I did, and your engine costs about $200.