It's probably a memory issue. If you're comfortable taking the side panel off, unclip and remove the memory modules. Then simply put them back in place, making sure they fully clip back into place.
I did double check the service manual from Dell. 3x beeps is a motherboard failure. So if reseating the RAM doesn't fix it, then your motherboard is shot.
So now it does 4 beeps? That's a RAM Read/Write failure. The manual states
Ensure that no special memory
module/memory connector placement
requirements exist (see "Memory
Installation Guidelines" on page 112).
Verify that the memory modules that
you are installing are compatible with
your computer