Hi all, I have a connection (and long layover) at Heathrow in a few days. I am starting in a hot climate and my final destination is a hot climate. I arrive in Terminal 3 and my next flight is from Terminal 4 (staying at the Yotel there for a few hours).

My question is - do I have to go outside to transfer between terminals? Is it a bus, or an indoor train? If a bus, can you wait inside and just run out to the bus, or is it out in the open far from shelter? And what is the general climate within the terminal? I'd rather not have to bundle up in winter clothes if I can help it, but I don't want to catch pneumonia either.

Thanks!