Invest. Say your home appreciates to $200K in 4 years, and you want to sell. It will be worth $200K regardless of whether you have $10K in equity or $200K. So putting additional money into your mortgage will not get you any more profit; it just saves you 4.125% in interest.
OTOH, the money you are putting into the market right now, today, has by far the largest effect on your future retirement 'stache. Actually, the money you have already invested has the biggest effect -- but as between today's dollar and tomorrow's, today's is more important, and tomorrow's is more important than the day after tomorrow's. Math: Rule of 72 says if you get around a 7% return, your investments will double every 10 years. So if you have $100K in the market today, that is $200K in 10 years, $400K in 20, and $800K in 30. OTOH, if you wait 10 years (because, say, you're paying off your mortgage first), you will only have $400K in 30 years -- half as much, because you missed an entire doubling.
I am a believer in paying off the mortgage before you FIRE, and in fact my mortgage is currently scheduled to be paid off by my planned RE date. But you have said this is not your forever house, so why rush to pay it off? Worse comes to worst and you do stay there forever, you can always take money out of the market to pay off the mortgage -- and you'll still have more money left than if you had put the money in the house to start with.