Hi , i have a question, which one do you recommend for adobe lightroom? IMac 5K retina 27” 2017 4.2 Ghz Quad core intel i7 16 GB 2400 Mhz DDR4 Radeon Pro 575 4GB Or IMac 4K retina 21” 2019 3Ghz 6-core Intel core i5 8GB 2667 Mhz DDR4 Radeon Pro 560x 4GB
I would stay away from 5K screens, as these make large demands on your hardware and often wind up lagging. And the GPU is now three years old, which doesn't help.
The 4K screen may also cause these lagging issues, depending on your hardware. The second computer listed, which has 8GB RAM may not have enough RAM. It also seems to have a less powerful CPU, which can also be a drawback.
So, I don't like either of them. But they may be fine.
Let me ask you a few questions: what camera(s) do you use? Do you do a lot of brushing and/or a lot of spot healing?