I buy the Rbpi 4 with 4 GB, I take the maximum to be used maybe for retrogaming.
I think 2GB is the best deal, 1GB is maybe not enough to load many sound/soundfont.
I put radiators on it and put in the official Zynthian box, need to make bigger hole for power and USB.
I can not say for the stability since I just use as an hobby, for my use, same stability as Rbpi3 without xrun problem on Pianoteq for example.
For the temperature, I have 42°C when just started, after 2-3 minutes without make anything 54°C.
Loading Pianoteq rise the temperature to 56°C, playing the midi demo on the gui with 2 effect attached rise the temperature to 65°C.
After remove all layer, the temperature is the same, around 60/65°C, I think a fan is needed.
For temperature problem, I read that there is some works make by Raspberry team with a new firmware.