I think you should just use 4 eggs. I don't think the difference in size between large and extra large is as large as 1/4, which is what it would need to be to justify another whole egg. You might add an extra egg white.
Last time I had this issue while baking a cheesecake, I just used 4 eggs and an extra white. My recipe calls for whipping the egg whites separately and then folding that into the yolk/sugar/cream cheese. It came out great.