Welcome to the forum!
I've dug out my letter from the CAA which says that all RT license holders are given "grandfather rights" at Level 4. So I take this to mean, without needing to do anything I have Level 4 already.
As you said, this Level 4 proficiency is only valid for 3 years. But the CAA also say that any flight test I do in the next 3 years will include the granting of Level 6 proficiency and subsequently require no reassessment.
I hope this helps!
OwenMc