There's an Arrivals lounge landside (after immigration and customs) at T4, it has showers and a hot breakfast in the morning. I think it also has limited spa facilities.
I think if you arrive at T4, then take the route to "Flight Connections" and stay in T4, you'll have to go through a dedicated security but no one will check your outbound BP. So that would be a way to get to the T4 lounges (and the spa). Once you go landside, e.g. to the Arrivals Lounge, I don't think you'll get back into T4 airside with T1 BPs.
[Or could one clear security at T1 then do an airside transfer to T4? I don't remember anyone checking our BPs when we did the T4-T4 sterile transfer, but it was <5am so maybe they did]
andrew