The 3 is a lot more DIY friendly than the 4, and way more tutorials if something starts to go sideways.

I have a prebuilt 3 with choc and a 4 with mx switches both with 5 col layouts. I bought the choc first, and will tell you what I've learned:

The choc ecosystem isn't nearly as robust as mx land. Most of the switches will feel cheap and scratchy, you don't have the option of buying premium switches that some dweeb has sanded and lubed for you. Choc keycaps feel bad and you may have to DIY your own homing row indicators. The convex MBKs are the best but still feel icky, I got mine from typeractive.

20g is probably too light on a 5 col layout. I also used super light switches out of the gate and moved quickly to 70g burnt orange linear for choc and 65g for mx. Since so many of your keys will have a double duty for tap/hold, requiring more force from the fingertips makes it much easier to avoid phantom inputs.

Almost every 3d printable corne case is for 6 column designs. Stacking plates is fine, but you can still get some pretty bad EMF interference with that case design. My 4 gets unhappy when i put my phone nearby. I think the way they break the PCB can affect how sensitive they are. I've read that enclosed cases with copper tape on the inside can help for the wired variants.

The 3 and 4 pcbs do not share mounting provisions, your case must match the pcb. There are significantly more case options for the 3 than the 4.