A tackle always has positional value. Today, the DL is a chess match. You put your best pass rusher against the weakest link on the OL. If that is your tackle, then you don't want trash out there. He is protecting your QB who *IS* the most valuable player on the field if he is a franchise QB.
It is also debatable whether anyone else in the draft is worth the 4th overall pick, yet you still have one.
First, let's not that Free wasn't bad or even average last year. He was actually pretty decent.
Second, they have two guys on the roster right now who one will likely be our RT in the near future - Green or Collins.
Third, the OL is already arguably the best OL in football with Free playing RT. For one to then use another first round pick on an OL, that player must take this OL to a whole new level. That's likely not going to happen with Tunsil. Not that he's not a great player but the fact is you really can't make this OL any more productive and good than they really are.
Therefore, as some have said, taking a 4th first round OL would be overkill and not getting true value in terms of improving the entire team.