Water changes appear to be ok. You still have residual NH4 in the tank from the water conditioner doing what it is supposed too. Your bio filtration normally takes care of this pretty quickly.
From what you have told me so far, I believe there is something wrong with your bio media, either it has somehow lost the bb through filter cleaning, big water water changes, unknown chemical, filters left off foe extended peroid of time. To have a 0 reading for NH3 with the seachem test shows that your water condioner is working, but to have a positive reading above 4ppm for NH4 with the api and seachem test on an established tank long time after water change indicates, something wrong your bio filtration, and that it has been building up over time. You won't have any nitirite readings thanks to the prime and big water changes you do. The fact that you do 2 good water changes a week and use a good water conditioner is probably the reason your fish are healthy.
For me in this situation I would treat it as having a mini cycle. BB not happening I would change 1/2 the media and start fresh with that media using a combination of seachem stability to have a safer speedy cycle, and seachem prime during my water changes. I would reduce the large water changes to around 20% twice a week until until the bio media has been properly colonized and finished it's nitrogen cycle (mini cycle has finished. )
Would be interesting for some other enthusiasts to give an opinion on what is happening.
but to 2 filters