Did you mean Kenalog-10 5ml (not 5mg)? Your description sounds like 10 mg per ml, in a 5ml multi-dose vial.
The code for Kenalog (not preservative-free) is J3301, and it is per 10 milligrams.
The physician injected 4 milligrams, or less than half of one unit. Normally in billing you round up at the halfway mark, and round down below that. However, everything I have found, including Medicare, says to round up to the next whole unit. So one unit of J3301.
If it was not regular Kenalog and indeed was preservative-free, or a single-dose vial, that's a different code.