There is no reason to switch to a 4-3, absolutely none. The system the Rams use is the hybrid 3-4 which is played much like the 4-3 as a one gap scheme. The difference is that in a hybrid 3-4 the DC has far more flexibility on how he uses his players. It's also far cheaper to run a hybrid 3-4 than a standard 4-3 as at least one of the edge rushers is an OLB who's cap number is far less than a DE. In the Ram's case, they actually run a 2-5 as both edge rushers are OLB's. Only AD and NT are true d-linemen. This means a team doesn't know if one or both of the edge rushers is actually going to rush or if they will drop into coverage and if one or two of the other LBs will rush.
This is why IMO the Rams can't re-sign Fowler who wants top DE money, not OLB money. It was a problem with Terrell Suggs back in the day in Baltimore as the WIL is played 85-90% as a DE not an OLB. IMO if Fowler will accept OLB money the Rams will be able to re-sign him. This is why I believe they will allow him to test the FA water first. If Fowler indeed leaves I believe the Rams will target an OLB with edge potential in the draft as they did with Ebukam if they want to stay scheme fit specific which it looks like they will do.
Fowler struggled when asked to play as a drop back OLB and Middleton struggled when asked to play man coverage. The ability to use players differently is why more teams are moving to the hybrid 3-4 and away from the standard 4-3. Staley has experience with the hybrid 3-4 and I think that was a requirement of McVay's.