Either way, whether your home increases in value, or decreases in value, the home you buy in 3-4 years is going to have a similar value swing.
So, let's try an exercise of a 50% swing, just for an extreme example. Also, I'll assume your future home purchase (Home B) will be targeting $300k in today's value. Your current home (Home A) is valued at $200k in today's value, and I'll assume you owe $110k on it now, and will owe $100k on it in 3 years. (No idea how right these are, but it doesn't matter)
Sell Option:
You get $200k now. What would you do with the $60k windfall? Invest it all to grow for a larger down-payment! (Worth $73.5k in 3 years with 7% market growth)
3-4 years later:
Your previously $300k target home...
RE Market tanks 50% prior to your buy, you could buy for $150k ($150k - $73.5k invested = Mortgage of -$76.5k)
RE Market balloons 50% prior to your buy, you would buy for $450k ($450k - $73.5k invested = Mortgage of -$376.5k).
Hold Option:
You stay where you are, and keep paying down the mortgage.
3-4 years later:
RE Market tanks, you sell Home A for $100k, owing $100k, netting $0k, and buy Home B for $150k (-$150k - $0k = Mortgage of -$150k).
RE Market balloons, you sell Home A for $300k, owing $100k, netting $200k, and buy Home B for $450k (-450k + $200k = Mortgage of -$250k).
Someone double check my logic.
Anyway, it looks like selling now gives you the wild chance of a lower mortgage later, but holding gives you more stability.