MEI A Level Mathematics Probability

hiya sorta stuck as to how to go about this q:

'When Charlie and Robin play a game, they toss a coin to see who goes first. If it is a head then Charlie goes first; if it is a tail then Robin goes first. Charlie and Robin play four games and each time the coin shows a head. Robin claims that the coin is definitely biased. Calculate a relevant probability, and discuss the validity of Robin's claim.'

any help would be greatly appreciated, thank you.