desktop extensible

Yes, there are 2 slots for memory in your system.  Since it currently has 4 GB on one slot, you can add another 4 GB to the remaining slot to achieve total of 8 GB memory.  Your system is shipped with an i3 processor, therefore, the memory speed will be operated at 2400 MT/s.  You will need another 4 GB memory module similar as this one https://www.crucial.com/memory/ddr4/ct4g4dfs824a/ct16369314.

This is the memory specifications for your Vostro 3671 https://www.dell.com/support/manuals/en-us/vostro-3671-desktop/vostro3671_mt_setupspecs/memory-specifications?guid=guid-5a94d12b-6cb0-453d-8a80-3d15d6040fe7&lang=en-us

And this is the validated memory configurations: