To what 4th?
Look maybe Robertson is another.
Maybe a new gk and no 6 are tge final bits.. maybe not.
For me with gomez not learning the positioning, Moreno if he plays, lovren, either gk and add in Henderson's wandering or can going to sleep... well thats 4 or 5 disasters waiting to happen. Sooner or later one does something and it's down to the quality of opponent to take advantage.
Vvd and Robertson might greatly improve things yes.
I've long said a new goalie is needed and no 6. If we have calm heads in these areas and willingness from the rest to sprint back while thry deal with the exposed situation we leave them in we can go well.
We sit too deep as well if under any pressure so we need that calmness and bravery to sit higher and keep playing.