I moved from dslr to mirrorless in 2017 and am now shooting with the A7r4. I would say that in terms of image, you would not see much if any real difference between those from your A99 and the A7r4. However, for me at least, the A7r4 has upped my game considerably. The ability to customize all the buttons, the silent shutter, focus magnification, immediate feedback on exposure and highlight warnings, metering for highlights, a bright image in dark conditions, higher burst rates, improved IBIS, horizon level, etc, etc, have proven immensely valuable for me in my work. I now know exactly what I am getting at the moment I press the shutter. I would estimate that I have at least 30% more keepers now than when using the dslr.

I originally has the A7r2, and find that the A7r4 has major refinements in handling and features. I would never go back to dslr. Whether the features I have described would really be useful for you is something you have to decide.

I will say that it took a couple of weeks of using mirrorless before I was really comfortable with it. However I now have it set up in a way that suits me perfectly.

Your camera is a 42MP camera with IBIS. It was Sony's top of the line model for the A-mount lenses migrated from Minolta and further developed by Sony. To 'see' a difference on a 61MP E-mount Sony AR7IV, you're also going to have to invest in Sony's top lenses that are native E-mount. Your best bet would be to test / rent a body and candidate lens and test for yourself for a week. There will be handling differences, but only you can decide is there are visual differences that justify changing from a camera that is already top-of-line (just older).