Originally Posted by JJeffrey

Yes, there is an underground tunnel that connects T5<>T4 airside, and a walkway that connects T4<>TBIT airside. It's possible and easy to walk T5 to T4 to TBIT all airside.

And the bus that connects the Eagles Nest to T4 is airside as well.

For lounges there are Admirals Clubs in both T5 and T4. The offerings are basically the same. T5 tends to be less busy than T4, though if they're departing from TBIT then the T4 club will be closer to their departure gate.