Mortgage and moving figures

I'm trying to get my head round some of the facts and figures regarding our mortgage and the possibility of moving house at some point in the not too distant future and was hoping for some advice.

Our situation:

Current house value (estimated using recent sold figures in our area) - £145-150K

Current Mortgage: £142K on SVR 4%. Portable mortgage.

We are looking to move in the next year or two and are trying to work out how much we need to have saved. We are looking at buying a larger house in a cheaper area (due to a possible job relocation). Income wise we are fine for the amount we would be likely to borrow its just a question of how big a deposit, how much for selling and buying fees and what kind of rate we can get.

So far I've looked at 90% LTV products but the interest rates seem very high compared to our current rate. Is it possible to move our current mortgage and rate to a new property (assuming we have enough deposit of course)? Am I right in thinking this would mean we would still owe £142K at 4% and we would need to take out new borrowing for any difference?

The houses we are looking at are in the region of £150-160k.

Thanks