Many things will have changed by the time T5 has opened. For example, we will finally see Delta, Continental, Northwest, and any other airline flying to London all flying into Heathrow. Where will they all go?
Honestly, I can see the following scenario playing out:
Delta, Air France, Alitalia, CSA, and Korean Air all move into Terminal 4, as they would need 8 gates at the most concurrently. KLM stays in T4, and Northwest and Continental come in. This would be another 6 gates or so. The rest of the gates go to the Air Malta (1), Kenya Airlines (1, and they are partners with NW/KL, so it makes sense to stay there) and Sri Lankan. The airlines in Terminal 3 would most likely be placed into Terminal 4 (selected long hauls), as well as Terminal 1 (since BA and BMI will have packed shop for T5 and T3 respectively).
As for the reason Star picking T3 over T4, I can think of two reasons. The first reason is gates, as there are 43 parking positions at T3, and only 29 at T4. Secondly, T4 is mostly intercontinental flights, while Star's operation at LHR is a lot of 737/A320 family (LH, BMI, Austrian). This could help account for it.