Add Samsung 870 EVO 4 TB SSD to Inspiron 3020?

Yes, the Samsung 870 EVO 4TB is compatible and will work in your Inspiron 3020 desktop.  Refer to the service manual and taking precaution while working on your system.

https://www.dell.com/support/manuals/en-us/inspiron-3020-desktop/inspiron_3020_sm/hard-drive?guid=guid-2cff108d-02dc-40af-bd5a-c4c22584a457&lang=en-us

You don't have to buy the SSD from Dell. So shop around for a good deal on a brand name SATA SSD of suitable storage capacity and with a good warranty.

What drive(s) do you have now, an NVME SSD or SATA HDD, and what storage capacity? That might influence what new drive you buy.

You might need an adapter tray for a 2.5" SATA SSD to fit in the 3.5" drive bay, if you PC didn't come with a 2.5" SATA drive cage installed.

Since Dell ships Inspiron 3020 with SATA Operation set to RAID, you will need to reconfigured BIOS and Windows to use AHCI before you install any new SATA drive. This has to be done the right way or PC may not be bootable:

1. At desktop, open Cmd Prompt window, run as administrator.
2. Copy-paste this command, which will start Windows in Safe Mode the next time you reboot: bcdedit /set {current} safeboot minimal and press Enter.
3. Restart the computer and tap F2 when you see the Dell screen to enter BIOS setup.
4. Change SATA Operation from RAID to AHCI.
5. Save the change and exit Setup. Windows will automatically boot in Safe Mode.
6. Open Cmd Prompt window again, as in step #1.
7. Copy-paste this command, which will start Windows in Normal Mode the next time you boot: bcdedit /deletevalue {current} safeboot and press Enter
8. Reboot and Windows will automatically start with AHCI drivers enabled.
9. Confirm that everything works correctly.

Now you're ready to install the new drive...

Caution! I just bricked my hard drive doing this. If you run Bcedit into safe  boot mode, the Microsoft OS will force you to enter a pin which you can't do because there's no internet connection. Infinite loop.