As long as they are standard MYW tickets, you can upgrade them to add a day. It will cost about $10 each. If they are separate tickets (not on a KTTW card as part of your Disney resort stay), you have to do it no later than the last day of the ticket. In your example, you could do it on Day 5, but not the next day. Also, it's always bet to use the tickets at least once before doing any upgrades (for complicated reasons that I won't lay out here). There is a Ticket Sticky at the top of the Theme Parks board with lots more details on upgrading.