We fretted about the exact same thing, arriving from the UK and having about an hour to catch the bus to Astorga. I prebooked as I have seen the slow queues on the ALSA desk at T4! We had hand luggage only, paid for our seat at the front of the plane to make sure we got off quickly, and power walked to passport control, then after that (depending on which gate you land at I suppose, but rule of thumb) you just keep walking straight ahead past luggage pick-up, keep exit doors to your right, go one level up, then out and catch the free, fast, transfer bus to T4, about a ten minute ride which looks like it's taking you out of the airport and causes quite a lot of stress for first- time passengers. At T4, get down to street level and the ALSA desk is by the exit to where the buses are. We have made it successfully twice and had time for the loo - there is one on the bus too, and they do a 10-15 min comfort break along the way - and buying some water and snacks for the trip. Having the ticket means you can choose your seat and get straight on, but beware the driver will insist you put your pack under the bus. Fingers crossed you will be fine (though I understand if you don't want to take the chance). Buen Camino!