Time from Arrival T-3 to Yotel T-4?

I will arrive at LHR around 9 am from EWR, and then depart at 18:00 to JNB. I'd like to make use of Yotel for a respite in T-4. I know I'll have to go through immigration, but how long should it take from arrival to get to T-4?

How early should I head back for my 18:00 JNB departure? I'm assuming my bags are checked through since it's one ticket with two legs, so I don't have to deal with it, right, and I won't have to check-in again - just get through T-3 security?

Thanks for help and advice.

Dirk