satti1999, 

Reseat the memory in the system. The 4 beep error code represents Memory read / write failure Memory failure 

Click the link below to download the user manual for instructions for removing/replacing parts within your computer.

Dell 3520 Online Manuals 

Below are your memory specifications for your system if you need to purchase replacements.

Memory module connector Two internally-accessible DDR3 SODIMM connectors
Memory module capacities 2 GB and 4 GB
Memory type 1333 Mhz (Sandy Bridge) /1600 MHz (Ivy Bridge) Dual-Channel DDR3
Minimum memory 2 GB
Maximum memory 8 GB