9.5 hours is a long time to be spending at LHR!
I understand from your other thread that you are limited by visa situation.
Basically, follow Flight Connections from T4>T3.
Check out what PP lounges you have access to in T3.
There's a fair amount of shopping, if you or your partner are so inclined.
You can also eat and drink.
But, boy, 9.5 hours ... enough said.