You can use the tickets from last year... one for one.
Or you can use the tickets from this year ... one for one.
So last year you got 2 x one day base tickets.
You can upgrade one of those to a 10 day park hopper
And you can upgrade another one of those to a new ticket.
You can not combine four tickets to make a 4 day park hopper.
You can use the four tickets individually four days in a row if you like.
Clear?
Knox