The SkyTeam LHR T4 lounge is wonderful. However, pretty sure you've got to be ST E+ (or the equivalent with a non-ST carrier, such as SV, using the lounge) or flying in business class. OP, do any of those apply to you? If not, you might be out of luck, as PriorityPass tells me the 4Deck lounge has closed, and I don't think there were any other pay-per-entry lounges in T4. Also, welcome to FT!