I'd look into a ZChiller. I've had one for a few years, the large one. It's amazing how well it works! It's also all copper, with convoluted internal tubing.
https://www.zchillers.com/
Looks like possibly mineral deposits.
I'd seriously consider a scrub/polish with barkeeper's friend, then rinse very well. Follow up by a thorough washing out with PBW and a sponge to agitate off the BKF residue. Rinse very well, then let air dry for a day or two.
Also: This is why PBW beats...
If you're planning on any kind of pressure, finding someone to TIG weld it for you is probably a better idea. Most time silver solder is used with a tight-fitting flared opening and that's going to be difficult/impossible with a 4".
With the amount of solids generally present in a Hefe, I'd almost guarantee that anything carbed at high volumes, even if the bottles hold, that they'd gush due to nucleation points.
My Baratza grinder finally showed up.
My god, what an eye opener. What a difference a grinder makes. No fines, very consistent grind. Same coffee, same grind size, same weight of coffee, same water. Only difference is the grinder. Cup is 50% better.