Carol,
If the holiday is after May there is a new Premier Inn opening which will be connected to terminal 4 with a covered walkway.
You can use free trains to link to other terminals from T4.
Currently they are taking bookings at a very good introductory rate if you can book in advance. Just paid £34 for a holiday in June which is excellent for Heathrow hotels.
On another note lots of the hotels along Bath Road (up to Harlington corner) are in the free flow area so service buses to and from the terminals are free, you can find a map and timetables online with a quick Google search.
Sometimes can get decent flight/parking deals with holiday extras but like said earlier train is often best bet especially if you can get discount tickets (usually 12 weeks prior to trip is best time to book)