Common plecs will get to its potential size which is around 18 inches... If its given the correct size tank to do so. Even if the tank is big enough (which would be 5-6foot) then it can still take years for the plec to get to their full size.
The problem with keeping them in tanks that are too small is they will get stunted. Meaning they will only reach a certain size before they stop growing. Although they stop growing on the outside, their organs continue to grow on the inside. Resulting in deformity and an early/earlier death
Whilst you might be OK with a common pleco in a 4 foot tank I personally wouldn't recommend it.
Bala sharks are definitely a no go in a 4 foot tank.... This is because of their potential size, which is around 14 inches but not only this, also they are very active fish and very fast swimmers. They need 2 feet of space just to get up to speed in the tank. Yes they can be kept in groups of 3 but the tank would need to be around 6ft long x 3ft wide x 3ft high... Although they will be far more shy and skittish if kept in small numbers. This is why it's recommended to have a group of at least 6 but preferably 10. A group of 10 would mean needing a tank around 12ft long x 3ft wide x 3ft high. This is why they are not a recommended fish to keep to the average fish keeper as you can see, not many people can give them the space they truly need.
Sorry, just seen your post saying you decided against the Bala's... Wise choice
Torpedo barbs are a much better choice although again they are need to be in larger groups. 5 or 6 would be a good number. In case you didn't know torpedo barbs are temperate fish meaning they need lower temperatures. Around 20-22 degrees c is good for them. Your 4 foot tank will be ok for them for a while although they will need a bigger tank eventually. Good luck