First of all, have you tried other brands of engines other than Thunder tiger? O.S Engines are really reliable and tuning is as easy as can be. YS makes some fantastic engines as well.

There are alot of issues when you try to run a 4 stroke engine in a helicopter. The 4 strokes do not run as many RPM's as a two stroke so the gearing needs to be modified. The entire power difference between the two engine designs requires a complete rethinking of the helicopter design.

I tried to run a 4 stroke in a Century Hawk several years ago. They made a conversion kit so you could run a OS FS-52 engine where a OS 32 was at. I never really could get the 4 stroke to run we enough to make it fly correclty. I live at 6500 ft and it just couldn't get enought air to produce power and when leaned out to produce power, there just wasn't enought ambient air to keep it cool enough to prevent overheating. I might not have had these issues if I lived at a lower altitude but that is not an option at this time.