Yeah with that footprint, you should really only have one common in there - there's not enough room for all 4 I'm afraid. That's why it's better to work out tank space for plecs by actual floor space rather than capacity - you could have a 100g tank, but if it's 20ft tall and only a foot square, it's no good for plecs - if you get me lol. You'd be looking at a 6 x 2 x 2 for even only 2-3 commons - that doesn't mean you couldn't have one or two more smaller plecs, just not commons - as like some other species, they're territorial with their own kind.
One common plec needs 4ft x 18" absolute minimum - which is just enough for a lower average sized common (12" or so). If he hit 18", you'd be aiming for a 4 x 2. Obviously the more plecs you get, the more space you'll need for territories. If I were you, I'd either rehome three of them and look into one medium plec of another species, or I'd upgrade the tank to a 6 x 2 x 2ft min. And then you'd have to keep a very close eye on it if you were to keep all four.
Sounds like a pain in the backside, but I (and others here) have multi-plec tanks, it's just with certain species you have to play it safe - more so with the bigger ones like commons and gibbies who really need that space! In my 6 x 2 x 2ft tank, I have two half grown commons, one rusty plec, one royal plec and two bristlenoses. I'm having to rehome one of my commons as he's too bossy with the other plecs - even though it's a big tank, just the way these bigger fish can be sometimes.
I'd rehome three of them if you're planning on keeping your tank long term and maybe treat yourself to a smaller but hardy plec, like a bristlenose or rusty
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/www.fishbase.org/Summary/SpeciesSummary.php?id=290
