Hi!
Four strokes are as easy to set by ear as any other engine. In pylon racing I think all seasoned pilots set their engines by ear! Have never seen one set their engine by a tach on the starting line.
A Tach is useful only when you file and modify a prop and want to compare to other props and try to reach a certain rpm.It's also good to have a tach when you want to set your engines on a twin to rew approximately the same (which isn't really necessary) before take-off.

If you haven't flown a four-stroke before... then for heavens sake try one out in another airplane before trying it in the Great Planes GEE-BEE R-2.
I have flown the GEE BEE R-2 now for 4 years and it is a very easy airplane to take off and fly....but, the landings are very, very demanding (that means difficult) to accomplish without flipping over.

Part of the problem is the high weight! Part is that the Great Plane GEE BEE R-2 wing is set +6 (stab 0) which means that on landing with a high angle off attack the wing stalls early (which means it will snap just before touch down).
The remedy is to build it light! a 20cc four-stoke is fine but not anything bigger (heavier). I have mine powered by a MVVS 15cc (.91) two-stroke with custom made silencer running a 15x4 APC and a Tettra 15oz "bubbleless" tank. All up weight is 5000g (around 11 pounds).

Before flying a plane like the GEE BEE R-2 it is good to try out several other fast flying/aerobatic airplanes so that you are used to what happens with high performance airplanes (Why not flying Q-500 racing). Other wise the first flight of the GEE BEE will not end so well...