Ah ok, I thought you meant he suggested ripping out the 12 already installed
I am 99% sure your 4mp cameras can have the resolution lowered to 2mp. So go into the settings, lower it, and see what it looks like with 2mp.
1) SnapAV isn’t bad quality, but is dealer only, ie you cannot find the pricing, he gets his mark up on the item and he makes money - I do not begrudge dealers for making a living. The Hikivision you can buy anywhere. He makes more money on product + labor to do the Snap vs just labor to the do Hikvision and he may not want to support a product he does not sell you so you’d be on your own whereas the Snap he may provide more support on the product (set up, firmware updates, returns if necessary, etc).
2) Re: 2 vs 4mp I am likely in the minority here but for security cameras the height/angle of camera placement is way more important than the MP. If the camera is too high up on the house and has the wrong FOV, 2 or 4 mp wont make out a face, a license plate, etc.
Though I mentioned you can just lower your 4mp to 2mp to test it out, there are also sites that will show side by side comparisons of frames at different resolutions.
Most of my security cameras at my house I’ve lowered the resolution and the frame rates because it is just not needed and I get more storage on my NVR. Some of the best rated affordable cameras are still 2MP Starlight Dahua cameras - they are supposedly the best night camera though they have a lower MP. Photography (not video) is a hobby of mine, and Megapixels are used as part of a marketing gimmick to get you to buy another new camera body. The size/quality of the sensor is almost as important which is why those 2 MP Starlight cameras are so highly rated. I forget but I think I once read the cover of Time Magazine used to be shot with the equivalent of 2 or 4 MP camera. This was years ago but still, it was a very widely circulated magazine!
I am basically not answering your question with my rant above on megapixels but with hobby cameras it can touch a nerve! going with Snap you’ll likely get better service and a happy dealer. If you are chasing Megapixels the Hikvisions are fine cameras but then you may be on your own to install/support, etc?