Yes very possible!
You will lose a little efficiency topping up, but for a bit of grain, not a big deal! The higher gravity the batch, the more challenging it will be. Consider a sparge in another pot or bucket as well to help efficiency.
You can also collect additional wort to top up your kettle after the boil begins so you can start w/ 4 and add wort slowly once you have a controlled boil after a few minutes.
With a little practice you can boil 4 1/2 in a five gallon pot, could also try fermcap, gas drops (simethicone) to avoid boilovers.
Nothing set in stone either about making 5 gallons...you could easily do 3 or 4 gallon batches as well.