re: Adding another return air vent

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have a 4 ton unit in the attic with a 3.5 ton outside if that makes sense

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It does make sense - it is not uncommon to have the indoor evaporator coil slightly over-sized for the outdoor condensing unit and it doesn’t affect the efficiency of the unit.

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currently only have a 20x20 return air vent in the hallway.

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I asked just to make sure they were giving you good information on the need for a second return. They are.

You should have 400 cfm air flow per ton (outdoor unit), so you need 1400 cfm (3.5 ton x 400). Your 20 x 20 return plenum is providing 800 cfm of return air to the air handler (20 x 20 x 2 = 800 cfm). So you are short 600 cfm (1400 - 800). The 2 is multiplied b/c the 1 inch air filter in your return grill allows 2 cfm of return air per sq in to pass through. A HVAC unit can only supply as much air out the supply registers as it pulls in from the return vent(s), assuming everything on the return side is sealed properly, so you are not getting the air flow you should from the 3.5 ton unit.

Looks like they would/should add a second return vent to supply an additional 600 cfm of air to the return plenum, that would be something like the equivalent of a 18 x 18 grIll. Looks like that might be a pretty sizable duct they would have to install to move that much additional air over 40 feet from the MB to the return plenum if that is what they are basing their calculations on.

Though I don’t really know if $1,400 is a fair price for what they are proposing, it certainly appears that a deficiency of return air is likely the source of your cooling problem in your MB.

Now, this might not improve your air flow problem b/c you are Indeed short of return air supply by a good bit, but you could try adjusting the duct work dampers in the attic to try to supply some more air to the MB - that is assuming you have adjustable dampers in your ducts, most do, but not all. Your damper to your MB is likely wide open, so you would have to partially close dampers in other ducts that might be supplying more air to those areas than needed to divert more air flow to the MB.

I actually have a similar problem to yours, but not as severe. I had planned to add an additional return vent when I replaced my HVAC in the next year or 2, so I haven’t obtained a cost estimate, but I’ve done my homework on what is needed.