I agree with the pay minimum on everything but 1 and then put your entire extra towards that. With a little work and excel you can work out what would be the best payment plan for you. Do Google search for Snowball Payment excel spreadsheet. This is a tool that can really open your eyes to whether your timeline is on track and your payment style is optimal. On some you can put calculate saving by putting it in as a negative interest rate rather then a positive. Both Mary Hunt’s Debt-Proof Living and Dave Bailey have good information on Snowballing payments and how it benefits you.
I don’t think they said down size they said down grade which means purchasing a van of the needed size but a little older or a little less fancy so the cost would be cheaper. Moving in two years is important because of schools and usually in the long run a better idea then private as the cost with 4 kids will be huge. Depending on the ages of your kids better schools might not be a big concern until they are moving toward later 3rd grade where they stop concentrating on the basics and enrichment starts to be a big advantage.
That being said if your timeline slips a bit you can always add enrichment until middle school which peer pressure gets to be big and making sure your kids are in a peer group mostly bound for college is important. Also consider the move more important for schools then it is for house size. You might be able to move sooner to a similar size house on a larger lot which you can add on later when you have save the money for the addition. Sun rooms are cheaper additions then bedrooms and the girls can share a bedroom for sleeping and use the sunroom for a playroom or finishing a basement is another option.