The train is not airside, so no, you can't do it. T3 is in the centre of Heathrow, T4 is on the south side, and there is no link. There is a bus that goes between terminals, but they quote up to 90min connection time, and what checks they do for people using it I don't know. But I would say it is a recipe for getting stuck, missing you flight for the sake of visiting a lounge really is it worth it? Along with wasting valuable drinking time!