Speedbird:
Sorry for your poor experience, unfortunately it's all at the hands of British Shopping Authority PLC (trading as British Airports Authority).
The local hotels all used to provide direct free buses to the terminals until someone at BAA saw an unexploited revenue opportunity, banned the hotel buses from the airport, and introduced their own so-called "Hotel Hoppa" service, which charges £3 each way. Having got this monopoly revenue stream they then contracted out the operation of the service to a local bus operator for the cheapest rate they could find - which implies the poorest service. I hope you didn't have to wait too long for it - there are stories of people waiting nearly an hour at the hotel and then finding when a bus finally arrives it is already full !
Please don't blame the bus company, by the way, they just do what BAA contract them to do.
The buses provided are of course modern disability-compliant ones with a low floor, as now required by legislation. The fact that the resulting overall arrangements are completely disability-hostile is of course quite irrelevant.
Of course it was all dressed up in terms of "reducing congestion", "reducing pollution", and any other politically-correct phrase they could think of quite unconnected with the real reason. In fact you could tell it was going to be a pig's ear when details of the new arrangements used the word "enhancement" in the title.
Then they realised that there was a lot of movement between the Central Area (T123) and T4 on the south side. They also realised there was a train service (the fact it was deep in the bowels of each terminal area and a long way from the terminals was completely irrelevant) connecting the two areas. So they stopped all the bus services they could and diverted people to the train. It adds on average half an hour to the previous time taken to get to T4. Indeed, the train only runs every 15 minutes.
Many people now get taxis from the hotel to the terminal, which is completely counter-productive.
One word to the wise is that the standard red London bus services in the immediate vicinity of the airport (which is out as far as the Renaissance) are free. So you can get a London bus from the bus stop outside the hotel into the airport at no cost (the driver just does not collect any fares). Possibly not practical in your case but useful for others. There's no publicity for this feature, which BAA hope will only be taken advantage of by regulars like airport workers, because unlike the Hoppa it costs them rather than gives them revenue. BAA pay an annual amount to London Buses for the facility.
If BAA had really wanted to reduce congestion and pollution they would have put a cap on passenger numbers. Which would of course hit their bottom line. So they have to pretend, while doing the opposite.