Obviously you can't plan on this, and the odds are really low, but if the stars align correctly, you might even be able to make the 9:35 bus to Astorga. The arrival time for the international flights varies widely, in my experience. I sometimes arrive in Madrid more than an hour early, and this is undoubtedly because the scheduled arrival time has a lot of time built in for delays. If that happens, especially if you aren't checking any luggage (which would be my recommendation for other reasons, but I won't go into that rant right now,
), if the immigration line happens to be short and fast, and if the shuttle to T4 magically appears when you walk out the door of T1, you might just make it.
I remember one year when I was on the bus from T4 to the Avenida de Americas bus station around a half hour after my arrival, so it can happen.
For the train option, since you're arriving in T1 I think that the metro to Chamartin is the best way to go. From T1 it will take you a total of 35-40 minutes. From T4, the Cercanias train is much faster. That train runs only once every half hour but it gets you from the airport to the train station in about 12 minutes. But if you add on your transfer time from T1 you are not going to save much if any time this way. Again, with an early arrival in the airport, you might make the earlier train.
But I think the more realistic possibility is that you will wind up on the 14:45 bus. :?