Yup. Keep the weight DOWN and put in a light weight engine with at least 18 pounds of thrust if you have the luxury. The P-100 is not that much heavier than the P-60 but it has some serious thrust. The P-70, I believe, is a fair bit heavier than the P-100 and drinks more gas than the -100 has a slower accel speed - it's not the perfect engine for this jet IMHO though it has been proven to work OK. The key to long flights and happy aerobatic routines on the A-4 is a punchy engine that is light and efficient, not a big engine that needs more fuel.

Vincent is the master of the whipped P-60 in the A-4 though, I would trust his advice if I were you. If you have to pick from a P-60 or a P-70, upgrade the P-60 to a SE model and whip it to the boosted specs. Just set the timer to 5 minutes or add a 12 ounce aux header tank between the saddle tanks (plumbed just down stream of the main) to deal with the excess fuel burn on the whipped P-60SE for a 6 minute timer.

I fly mine with a Wren SuperSport and the 12 ounce aux header tank. At 18 pounds of thrust, never do I really long for more power - it seems it has the perfect balance of strong vertical, assertive takeoff and low fuel use. I set my timer for 7 minutes and always land with the full aux tank plus a few ounces in the main. Sometimes I take off with half saddle tanks and set the timer for 6 minutes and land a lot closer to half aux tank, the 6 to 7 minute flights are fine with me and the lighter weight is more fun in the first few minutes of flight. But the big thing about the Supersport is it really can sip fuel for the thrust you get.

I've flown Tam's A-4 on electric power at 3 pounds lighter weight. In the pattern it makes all the difference. This thing is so easy to fly when light, though it can't compete with speed and vertical, the EDF shows what a heavy bird is missing in delightful handling and scale speeds on approach.

You also want to watch the speed on the A-4. I don't think you want to take her too fast, limit it to 190-200 (of course 200!) - but for real. The tail can wag a little in gusts, I'm not sure what that airframe is really stressed to do but I beefed up my tail a little in the rudder hinge area just to be safe. I'd hate to see what would happen on that big A-4 rudder if you over sped and got some flutter. It could be extremely ugly and dangerous in about 0.002 seconds, but plenty of guys are running it safely and repeatedly at legal speeds on fast P-60's, Supersports, P-70's, etc. A stock P-60 is not going to take this A-4 above 160 straight and level IMHO so you are safe there, but getting to that speed is gonna take 1/3 of your tank!