Hi Andrei,
The first result in Google was this:
http://www.crucial.com/usa/en/compatible-upgrade-for/Dell/inspiron-15-%283551%29
It mentions that the memory in your computer shouldn't exceed manufacturer supported memory (which in this case is 4GB).
May I ask you why you wish to upgrade your ram? Have you checked if your memory usage? Personally, I don't think it would make any sense to spend money on memory, that wouldn't be used anyways.
If you consider your laptop's performance to be sluggish, then you're better off spending your money on an SSD instead of your 500GB SATA HDD.
Hope this helps