Crikey, how long does that take? Do you use a python or python type system for water changes?
Always takes me longer than I expect to do W/Cs. It took me about an hour and forty five mins to do a 50% water change and gravel vac on my 57 gallon yesterday! That was with thorough gravel vac, moving decor and stuff, and to be fair, a lot of that time is waiting for water to boil to temperature match the water... takes longer in winter to boil enough water for that many buckets. It's a bit quicker in summer. I'm also easily distracted, spend time faffing about with plants and cleaning glass and stuff, so that drags water changes out more. Think it would take me a long time to do all four tanks in the same day and would kill my back! If you're using buckets, go steady on your back - make sure you're lifting properly, and don't overdo it for no reason! It's easy when you're young not to worry about your back, but you pay for it later in your 30s and 40s...