Yes, you are correct. Because you are departing from T1 you will have no access to T4 departures. I would use the Arrivals lounge then the Terraces in T1. But that's a very long wait for a connection at the airport and I wouldn't fancy spending 9hrs in a lounge. If you don't have much hand luggage and your luggage is through-checked already why not make the most of the day and see London?

Heathrow Express connects T4 with T123 every 15 mins, free of charge, landside.