How to connect and use a Mac mini 4 and Mac mini 2 together

No.

Migrate your data from the M2 to the M4.

Transfer to a new Mac with Migration Assistant

Then setup the M2 for another family member to use or sell it.

What to do before you sell, give away, trade in, or recycle your Mac

It depends what you want to do. If you want to be able to see files on the other computer and share peripherals then as long as they are on the same network then you can set all that up in the Share menu in System Settings. If you're going to do that then you need to read up about sharing and security because some of the things you share will be shared with all devices on the network.

If you want to connect them together to share workloads for heavy processing then I don't think you can do that. If I'm wrong then I'm sure that someone who knows more about it will be along sooner or later.

Re: “If you want to connect them together to share workloads for heavy processing then I don't think you can do that.”

There are programs that can do that, but they must be designed specifically for a distributed computing environment. (Think weather models, or SETI @ Home.)

Most ordinary applications like Microsoft 365/Office or Adobe Photoshop are not designed to

work in this way. Many will not even take advantage of all of the cores available on a single machine, most of the time.

You can connect them by signing in to the same Apple ID. Then, you have to go to Settings → General → AirDrop & Handoff and enable Handoff. Make sure your Macs are connected to the same Wi-fi and Bluetooth is on. Then, tap 3 fingers on one Mac’s trackpad, and move the cursor to the edge of the screen. You should then see the cursor on the other Mac. Now that the Macs are connected, you can drag apps to the another screen. Just make sure the another Mac also have the app.