Quite happy with it, coming from an A7Rm2. Biggest improvement for me is the option to toggle peaking with a custom button. I was a bit afraid of the bigger 60 MP file sizes combined with stories about less favourable high ISO performance. The files are big indeed and I want to shoot uncompressed raw which gives you a whopping 120 MB file; these days I convert them to DNG to get to about 60 MB which is just about tolerable.
High ISO performance is a wash between the R4 and the R2 as far as I'm concerned; some reviewers moan about somewhat less dynamic range but we're talking differences of 0.5 stop or so which I feel is irrelevant. Jim Kasson says essentially the same, with more authority than I can do.
Autofocus performance is a big step up from the R2; just this afternoon I went out shooting about with the Sony 2.8/20 I acquired and I got comparatively few misses. Mind you, it's not an A9 but for me it's good enough.
Finder is great, as are the bigger battery, the joy-stick, My Menu and a number of other features added. Don't really care for the second card slot. Felt no need for a battery grip so far, even not for handling the FE 4/70-200. The R4 is advertised as fairly weather-resistant so that's comforting, I don't put away my camera when the first drops of rain are falling and not even when it's seriously wet but I make it a point to not make that last too long and dry the camera every now and then. Sony FF cameras are renowned dust magnets and the R4 is no exception. Before almost every outing I blow out the sensor chamber with a rocket blower, but I change lenses on the road as I see fit. However, the camera's throat is only open for a few seconds when doing so and I keep the rear of my lenses clean. After 7 months I haven't had to wet-clean the sensor yet, my shutter count is at 6881 now.
Would I rebuy it? Almost certainly yes. Main reasons to stick with Sony are that Zeiss Loxia and Batis lenses only come in E-mount and switching gear is expensive when you possess over 10 native lenses. While there may be nicer cameras around like the Nikon Z7 and Canon R5, I'm not really tempted to switch, the camera just works and lets me do what I want to do. I really want to have a do-it-all camera and the R4 comes pretty close in my book.