Capping rate is the maximum fare you will pay for a day with your Oyster card... The capping rate is actually 50p cheaper than buying a 1day travel card from the ticket machine...
For Zone1 to Zone6, the capping rate is 6.2 while Zone1 to Zone4 is 5.9
If you are going to do a huge amount of travel within London, then you may well buy a Weekly Travel Card... And the travel card will be fitted inside your Oyster Card
Then if I going to stay in London for 4 days, and travelling within zone 1 and zone 2, what type of ticket should I buy from the counter? The day pass or what?
btw, is there any "overnight" route travel from London Central (Victoria?) to Heathrow Airport?