XPS 15 L521X, screen is dark, what could be wrong?

I believe that 4 beeps is memory failure.

1. Disconnect all peripherals: printers; USB devices; media cards; USB hubs and extenders; etc.
2. Disconnect the AC adapter.
3. Disconnect the battery.
4. Press and hold the power button for 30 seconds to discharge any residual electricity.
5. Use a soft pencil eraser to clean all the contacts on each RAM module.
6. Moisten a microfibre cloth with 99% isopropyl alcohol and use a credit card to slip the damp cloth into each RAM slot to clean its contacts.
7. After everything is dry, reinsert one RAM module, according to the separate tests below, firmly at a 30° to 45° angle, ensuring that it seats fully in the slot.
8. Reconnect the AC adapter
9. Reboot the laptop or enter BIOS to see if RAM is recognized.

If you still get the error, you must diagnose whether it's an issue with a RAM module or a slot, by performing four tests, testing just one module at a time:

• RAM module 1 in slot A
• RAM module 1 in slot B
• RAM module 2 in slot A
• RAM module 1 in slot B

If module 1 works in slot A and B, and module 2 does not work in either, then module 2 is bad.
If module 1 and 2 work in slot A but neither works in slot B, then slot B is bad.
etc.

Before each test, follow steps 1-4 again. If none of those tests gives you a working system, then your motherboard may have failed.

Questions: When you say that you replaced the RAM, do you mean that you removed it and reinstalled the same RAM modules? Or, did you upgrade to different RAM? If it's the latter, then perhaps the new RAM is incompatible with your system.

Whenever touching components or working inside a computer, wear a grounded wrist strap, also called anti-static wrist strap, ESD wrist strap, or ground bracelet. It's a cheap and sensible precaution. Rest the laptop on an anti-static mat or at least a reasonable alternative such as corrugated cardboard. (Repeatedly touching a metal case part is not sufficient mitigation with modern components.)