Depends if you have a rack with cooling or not and how deep the rack is. I use a 4 RU rack for 3 units, then I have approx 1,4-1,5cm space between devices and to the top. To the bottom there is lesser space.
I am using Thomann Eco II racks with 23cm depth, thats good for the recording interfaces.
https://www.thomann.de/de/thon_rack_4he … ompact.htm
I use a little deeper racks for external FX like Lexicons.
https://www.thomann.de/de/thon_rack_3he_economy_ii.htm
Four devices in a 5RU rack would leave IMHO too few space between devices.
I would use either 2x 3RU for 2+2 devices
or
1x 4RU and 1x2RU for 3+1 devices (then you can split Firewire 400/800 / USB devices)
or
if you want 1RU reserve: 1x 4RU and 1x3RU for 3+2 devices
BR Ramses - UFX III, 12Mic, XTC, ADI-2 Pro FS R BE, RayDAT, X10SRi-F, E5-1680v4, Win10Pro22H2, Cub14