Thanks Patrick

Do I need to book an advance ticket for travel from T4 to London Kings Cross (via tube) also? Or, does the cost remain unchanged as it is not a railway train and is a metro tube connecting central London? I believe it is GBP 5.50.

Cost remains unchanged. Just buy it when you arrive at the underground station.

If you get an Oystercard when you arrive at Heathrow and put some money onto it (say, the £25 Patrick suggests) the fare to KX debited to the card at 1015 or thereabouts will be £3 (you simply press a loaded card onto the gate at the platform entrance).

Buy a paper ticket from the seller or a machine and it'll be £5.50 (the "paper" fares are deliberately absurdly high to encourage electronic payment). The rest of the money you've put on the card will still be available when you return to London

Thanks. Is it like minimum recharge to oyster card can be £25 only? Also explain how oyster card can be used during a day full of travel within London. Does it carry an upper cap and lower cap for a day use?

http://www.tfl.gov.uk/tickets/14870.aspx

all the rules for oyster