Oyster is not some sort of travel pass and therefore there's no particular price tag for it for 4-days travelling.
Basically it's a card that you have credit in it, and the price of ticket, depending on where you've travelled, would automatically be deducted from the card as you tagged the card when you enter/exit the public transport system.
There is however a daily price cap, ie that will be the absolute maximum you'll pay per day, for using the public transport. If you use less than that, then you're charged only for what you use.
The current daily cap for zone 1-3 travel regardless on bus/tram/tube/DLR/overground is £7.50 for peak travel, and £5.40 for non-peak travel. (http://www.tfl.gov.uk/tickets/faresandti…
Therefore, I guess the "max price", for 4 days, will be your maximum expenditure for public transport i.e. £30.00 and anything below that would be the best price possible according to your usage.