This is a thread about using the CNC Shield v4 as the main board. If you plan to do this, buy this kit:
https://www.aliexpress.us/item/2251832035811475.html?spm=a2g0o.detail.pcDetailTopMoreOtherSeller.10.3fe9i0iWi0iW2v&gps-id=pcDetailTopMoreOtherSeller&scm=1007.40196.366991.0&scm_id=1007.40196.366991.0&scm-url=1007.40196.366991.0&pvid=5c83e78e-97fb-48de-a5c7-35db0da5b120&_t=gps-id:pcDetailTopMoreOtherSeller,scm-url:1007.40196.366991.0,pvid:5c83e78e-97fb-48de-a5c7-35db0da5b120,tpp_buckets:668%232846%238111%231996&pdp_ext_f=%7B%22order%22%3A%2210%22%2C%22eval%22%3A%221%22%2C%22sceneId%22%3A%2230050%22%7D&pdp_npi=4%40dis%21USD%2119.99%2119.99%21%21%2119.99%2119.99%21%402103010e17397411430922198e1202%2155305829959%21rec%21US%216242698492%21X&utparam-url=scene%3ApcDetailTopMoreOtherSeller%7Cquery_from%3A. This may be more expensive then the $2 "cloned" boards, but it is entirely worth it. The cloned boards have many problems that require cutting PCB and soldering to fix. To find out more about this, look up "Fix Cloned CNC Shield" and an instructable should show up. The expensive Keyestudio kits work right out of the box, and come with the required stepper drivers. If you need any help, just ask.
Here is the pinout diagram for the CNC Shield:

Here is the pinout for adding a buck converter powered servo:

I will probably not update this thread much (or whatever the posts will look like on the github) because I am busy developing my own budget MMU (for klipper printers only). That's it for now!
Liam
I have it working (in breadboard mode) with a CNC V4. Pins for my V4 are in the image below.
Micro steps will depend on jumpers on the V4. Had an issue with the steppers and how they are wired with the flat cable that came with them. Had to swap some pins. Easy to check what pairs to use by shorting stepper pins together to find the ones that cause the stepper to 'turn hard' by hand. direction can be changed by swapping the connector on the board around.
No on GRBL. You will overwrite what might be on the nano.
the code above has the switch connected to A3 (and ground) I had a 3 wire patch cable connected to the switch and use 2 of the connectors to connect to the V4.
be aware V4 boards are not all the same. I ended up with this one
Free Shipping! New! Keyestudio Cnc Shield V4 +3pcs A4988 Driver+ Nano Ch340 For Arduino Cnc - Integrated Circuits - AliExpress
I'll get some pics later.