i also like these brands of hop ups:

Knack racing
five star racing
Hiro racing

personally don't bother with 3 racing it is shyt. the pulley adaptors deform, and all moving parts are of inferior quarity.

skyline is good, but i would only recommend it for non moving parts, and i also would noy recommend the large rear support for the radio tray, as it requires bolts, and doesn't have screws for the battery tray built into the support. subsequently it is annoying to attach these screws with glue, and they inevitable come loose.

my top list of option parts makers are;

kawaharra, knack racing and 5 star. equal pegging all parts are exceptional.

i had a major crash with my mtx4 where i bent the rear engine brace litterally into a "Z" shape!!!!!, yet my knack racing 4mm chassis is only barely tweeked. it rises up only by 1mm.

-hiro racing, good quality but minimal hop up products for 4
-skyline ok but not japspec