A 210hp 327 has a pretty small cam in it. .
The 210 has the same -929 cam that the 327-275 L30 has, as well as a great number of other low and medium performance 327 and 350 engines, they used this cam a lot, and it will produce 300 HP with the right combination of other parts.
The "choke point" in the 210 is the heads and 2 bbl carb. The heads are lousy and should be changed. Changing them will give a remarkable performance improvement.
Switching to a 4bbl carb should have given you an improvement tho, a pretty good one even. It certainly, by itself, did not make an engine that wound up just fine suddenly not want to rev over 4k...
Since you made a change, go back thru everything you did. If the vacaum is steady and high, you probably don't have a vacaum leak. Check some plugs, how do they look? Do a WOT pull, shut it down at the top of second, get her safely stopped, and look at the plugs again?
Checking the float levels on the carb is an excellent idea. While the top is off, make sure the secondary flapper moves freely, blow thru the secondary jets, make sure the passages are clear (be VERY careful of flying gas, maybe pull the carb and dump the fuel before doing this!)
To prove it is a secondary fuel delivery problem, you could disconnect the link that operates the secondary throttles and drive it on just the front 2 barrels. It should pull smoothly to whatever was the max useable rpm with the old carb and manifold.
With the checks you allready made, I'm going to assume for the moment that this is not an ignition/timing problem.