That's the reason to start low and work up till you find the sweet spot for a round when reloading. Once you find the sweet spot for a round go with the lightest charge because more powder isn't going to improve accuracy .

You say it shot the same with 4.2 4.4 4.6 4.8, I'd go with the 4.2 for range use, lower recoil and to save on powder. If I was using them for self defense maybe 4.8 for added velocity.

My favorite load for the 38 is with a 158 gr. LSWC . I just think the 158 gr. bullet is a great match for the 38 spl.