Well, thanks to all your advices the guy came out last week and exchange the piston! to match the 4 t outside unit. And the AC actually cooled the house during the day to about 75/76 F and was able to hold it!
BUT! today it's right back where it was at 80 F and hasn't moved down from that in hours!!!!!
Uggghhhh I am so frustrated i called him again and he says he now has no more clue whats wrong. He said maybe its leaking pressure and he didnt tighten the piston good enough.
It is extremly hot today but I think it was in the lower 90's also Sunday... so that couldnt be the problem that the AC just can't cool it down below 80 F as far as i understood all you guys, right?
I think i need a 2nd opinion on my unit before i lose my mind.
What i still and again don't get why did it work just fine for 5 days with the new piston cooling down during the day to 75/76 and now stops again. Any clue?