IMHO, this entirely depends on the TV you have, how much money you're willing to spend to improve picture quality, etc. Not only if it's 4k, but how good the upscaler and motion processor are when operating in 1080p and in 4k (could be very different). If you have a really good 1080p->4k upscaler in your TV, you might decide the upgrade to not be worth it; but if your upscaler isn't so great, then letting the PS4 Pro handle it could be worth it depending upon the upgrade cost (i.e. selling your PS4 or trading it in for a PS4 Pro).
Your TVs pre-processed motion scaling, artifacts, color depth and gamma while operating in 4k are all important here; so if you really like your TVs 4k WHILE IN GAMING MODE (or PC mode for no postprocessing) then it might make sense to make the upgrade.
It might also be worthwhile to download some footage of gameplay from the PS4 and PS4 Pro and to play it back on your TV at 1080p, 1080p->4k upscaled, and native 4k... To see how the imagery looks on your TV.
Lastly, does your TV do 4k HDR well? If not, then IMHO, it's not worth the upgrade. If so, then that's something you just can't get from the PS4. FWIW, I am a huge proponent and fan of HDR and Dolby Vision, and think it's a bigger qualitative improvement in picture quality than what you get going from 1080p to 4k.
All in all though, from what I've seen, the Xbox One X is more worth the upgrade since the PS4 is already rendering native closer to 1080p, while the Xbox One was rendering most titles closer to 720p; so the scaling difference on a 4k TV is noticeable between the two consoles. So, in effect, if you asked "is the Xbox One X worth the upgrade?" .. then yes, it is. But is the PS4 Pro from a PS4? ... Kind of, but the argument isn't as clear cut since you're not going from ~720p->~4k but ~1080p->~4k; and FWIW, the Xbox One X does 4k better; so again, the upgrade path is more clear.
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FWIW, if you know someone with a PS4 Pro that you can borrow, or, if you can get your hands on one for a week; you can test it yourself and see if it's really an improvement, visually.