I have one slight concern about having a 12MP DSLR in 2018, and the concern is that 12MP will become obsolete or near-obsolete by 2020.
Look at the images on the internet which were made with a D3s. Are they good enough for you? Assuming it continues to function, it will still make images that look that way in 2020.
That's almost like $1,200 down the drain.
Purchasing
any camera in the belief it is a long term investment is not a good plan. Electronics in general are not the sort of device one should expect to give decades long. service.
I was considering the D4, as it's relatively newer. But it isn't as good in low light
Is that important? Do you even make many photographs in reduced illumination?
Will the D3s still be good in 2020?
ANYTHING digital has a relatively short useful life. Digital cameras should be regarded as consumables. D3s or D4 either one, as they age they will begin to exhibit failures that cannot be corrected because no more service parts are available. The D3s and D4 are close enough in time that I suspect you're not likely to get much benefit in this regard by selecting the newer camera.
Yes, 16 mp is bigger than 12 mp. Not much. Not enough to care about particularly, but yes it's bigger. Given your stage in life I can't imagine you are going to be in a position to print a lot of images of any size, much less large enough to make a larger sensor worth the money.
Yes, the D4 is an excellent camera. So is the D3s.
If you are looking at D4's, take note of what memory card format is installed in the particular specimen you are considering. Some are configured for XQD cards, which are way more expensive than the cards used in your D3s.
What
specific features have you
actually identified in the D4 upgrade that you need and are not present in the D3s ? If you don't get another single thing from this response, take not of this question at least. It's
very important. If you don't have a good answer to this question you don't need to change cameras.
Incidentally, just as a reality check,
I happen to still have a D3s myself. I also have a D850 and two different medium format systems. The point being I can upgrade to a later pro body if I so choose, I just haven't found a good reason for doing so.
