I know it comes down to more than just cost but there’s only a £600 difference between the A7R3 and A7R4 so it must be worth it going the extra bump in price?
The extra cost just doesn't apply to the camera.
More pixels means larger files, so more storage space required.
Larger files means longer processing times in Lightroom and Photoshop, even on a fast pc it takes time to work on those files.
This also means faster SD cards are a must (I don't used anything less than the 300MB/s read/write SD cards - and you'll only fit 501 images on a 64GB card). Depending on what you shoot you'll spend time waiting for the files to write from the buffer to the card, I shoot portraits and wildlife along with landscapes and if I burst then I have to wait for that buffer to clear.
The gains from the 42MP to 61MP aren't huge - yeah you're getting more pixels but this is only really beneficial if you're going to massively crop and print large (lets say anything A1/A0 or larger on a regular basis). The RIV also has massive noise at high ISO because of all those extra pixels you have.
The ergonomics of the RIV are much better than the RIII, with regards to the grip and the buttons on the camera. The battery life is pretty much the same with both (i.e. excellent). The weather sealing is better on the rIV - I've dealt with sandstorms in Nambia and extreme rain in Iceland.
But there are a lot of times I would have preferred I'd stuck to the A7RIII, it's more than capable
Oh, and if you want to do astrophotography the A7III beats both the RIII and RIV - but astrophotography with a Sony is by far it's weakest ability.