3 bed with potential to be proper 4 bed, nought wrong with that. Can you do a comparison to see what 4 bed in the same road goes for?
The question by the sounds of it - is do you want to move? When doing your calculations don't forget to factor in stamp duty on your purchase, agents fees on your sale, moving costs, survey costs, fees for new mortgage. Would this swallow up some/all of your equity ?
If you have space, could you get a lodger to help pay the mortgage ?