To the person who said don't use paper tickets as they are much more expensive than oyster, I'm sorry, but you're wrong.
If you go to a ticket office and ask them their advice, ANY clerk will tell you this same thing
If you are only in London for four days, then I would avoid getting the oyster, unless you plan to visit regularly.
To get the Oyster, you have to pay a £3 deposit, which is refundable if you give it in at a ticket office.
Then, you have to charge it with money. Assuming you do not travel before 9.30am on mon-fri, the maximum charge for travelling in zones 1-3 is £5.40 (only 50p less than a paper ticket), but this is NOT A TRAVELCARD. Providing you register an entry and an exit for every journey, you shouldn't pay more than the maximum 'cap', however, if you do not register at one end of a journey (eg. you can't find a reader, the reader is not working, you go through a busy station and pass your card over the reader too fast for it to register but manage to get through behind the person in front - it happens all the time, etc) that 'unresolved' journey will cost you £4, and does NOT get included in the 'cap' rate.
Also, if you DO travel before 9.30am mon-fri, the max capping rate for zones 1-3 is £7.50, even if you start at 09:29 and 59 seconds.
Also, you didn't say what services you would be using. If you plan to use national rail (i.e anything other than the tube and bus networks) then you can forget Oyster completely, as they are not valid on most of these services yet, unless you have a 7-day travelcard (£28.40 - too expensive).
So you see that you could end up spending a lot more than a paper ticket.
I would recommend, for simplicity, to spend that extra 50p a day and get paper tickets - a lot more reliable, easier to use, and gives you more options.
The ticket office WILL sell you an oyster for just four days, if you really want one, but they'll try and talk you out of it, since it's not generally the best for visitors.
7 years working in a ticket office.