Defenses don't really do anything most of the time anymore if you're speaking in general terms. They try to match up on personnel along with down, distance and game situation.
When in a four man DL they are generally going to shoot the gap from an under position on a passing down. You may have 1-2 players, generally the DTs, shoot and hold at times which does have gap control. There is by definition no gap control when a player shoots the gap.
We play more nickel as a base with our LBers than not but this is again a game to game as well as play to play situation. As others said we don't really play a Tampa 2. We do use our LBers to fill the holes since the DTs are often shooting the gaps. Someone has to be at home as teams are going to run to daylight to those unoccupied gaps.
This is important since many here don't understand the role of the DTs in our defense and wonder why the RBs are running to where they aren't. They think the DTs are being driven out of the hole when in fact they are leaving the gaps a fair amount. And of course they do get blocked at times, too.
In the 50s Paul Brown began to split his OL further apart and started using early ZBS techniques. RBs lined up deep and ran to where the defense was taken or 'holes'. Supposedly this is where the term run to daylight began. It gave the league fits and the only team to beat it much were the NY Giants with one Tom Landry as a safety. Tom later developed the Flex Defense to defeat the Browns and other teams which copied Brown and defeat the rushing attack. Tom also tweeked the 4-3 (he didn't actually invent it BTW) into what it was for a very long time and what many people still envision it as.
Our LBers fill the holes given up by the DL. But we aren't in a true 4-3 as much as 50% of the time. We run all kinds of pass coverage from the Nickel, Big Nickel, Dime, and Big Dime depending on the offense, down, distance, and game etc. Seattle and teams copying them use all kinds of 'zone' coverage involving DBs and LBs as well as even DL. Teams roll coverage and use inverted and regular schemes with diamonds and blocks of coverage. It's way more complicated than my rudimentary knowledge.
There are books on all of this and plenty of internet fodder out there.
So in essence there is no simple answer to the OP's question.