Actually a 52-size 4 stroke from Saito or OS is mostly seen in scale planes these days. For a 40 size 3D plane the best 4 strokes are YS-63, OS-70 Ultimate and Saito to 72/.82, and they weight about the same as a 2-stroke 46 ( with mufflers ). A quite good and inexpensive 2-stroke choice is the OS 46 AX with a 12.25x3.74 prop, or a good 52 maybe ( I have no experience with Irvine or Magnum though, some say they are as good as OS, others notice some issues with carb and run reliability ).
But we seem to hijack the thread...An inexpensive ARF plane has the added benefit of shorter time to fly and less grief if it meets the ground the wrong way ;-)

Smok