From my experience...
RailAir is likely to be significantly cheaper than travelling via Paddington and using Heathrow Connect or Express. Incidentally, if you buy a ticket to Reading and then the RailAir ticket separately on their
website you'll save yourself a coule of quid. I have used it a good few times in the morning rush hour from Terminal 5 and never been delayed getting to Reading.
As for check-in times, technically they tell you to be there three hours beforehand but that's mostly to scare you: I would always aim to get there some time around then but don't get too worried if you're delayed en route and end up arriving more like 2 hour or even 90 minutes ahead of time. Those should still be fine.
T4 is a bugger to get to these days: you could either take the RailAir to the central bus station then either the Underground or HEx to T4 (both of which are free within the airport)
or jump off the RailAir at T5 then any of the local buses that go to T4 are free within the airport perimeter. Since T5 is the first stop before the central bus station the latter option may be quicker. RailAir have a page on their website specifically telling you how to transfer to T4 here:
http://www.railair.com/terminal4.phpJust to clarify slightly about transfers to T4 being "included" as part of your ticket, it's actually the case that
all travel within the confines of the airport by London buses, Underground or HEx is free of charge: you won't need to show any kind of ticket to use it.
Edited to correct mis-reading of original post and add info about T4 transfer
