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I have a great 1926 conn (5j?) with good false tones that are easy to lip into tune. The only problem is these false tones are not real loud. So would a 4th help increase low end volume? It has a small shank lead pipe right now so what are the pros/conn ( pun intended) of putting in a large shank lead pipe.

How is it right above that, for example Bb 1 & 3? My slim experience with this is, my 1926 Conn Eb tuba (actually a Pan American Eb Giant Bass) was terrible on Bb. There was no real support there, you just had to buzz that note and hope it was audible. If I'd had 4 valves, I sure wouldn't have expected anything better below that. But on a tuba that enthusiastically responds to that note, I think you could more reasonably expect a 4th valve to be some use. Whether it would be worth it or not - or possibly even ruin a good thing - I make no prediction.

If there are no issues with the instrument other than false tones that don't project well, consider a small shank tuba mouthpiece. If there are other challenges (intonation) then changing to a normal shank receiver/leadpipe could help or make things worse.

I have a 1918 Conn Eb (~20 inch bell) 3 valve tuba that the low Bb was squirrelly at best. Damage to the lower tuning slide crooks on both 1 and 3 was the cause of this on my instrument. After repair, that note is OK (not as good as a Bb on a BBb tuba but playable).