If you have time at Paddington, walk as far forward along the train as you can before you board. The exit from the platform to the terminal at T4 is at that end of the train.

I don't know whether "next weekend" for you means 3-4 September or 10-11 September. If it's the former, it's a Tube closure weekend (Piccadilly Line partially shut, disrupting services to/from Heathrow). The alternative arrangements include the acceptance by HEX of London Transport Zone 1-6 Travelcards for standard class (Express Class) travel. (London Transport tickets are normally not valid on the HEX.) One of these Travelcards should be substantially cheaper than a single ticket on the HEX. So if you're travelling this coming weekend, don't pay more than you need to.