Macbook pro - do I need to upgrade my memory?

I have been advised by apple support that if I want high sierra I probably need some more RAM. I use a macbook 13"pro late 2011. The system was sluggish on high sierra. I have now had it downgraded to el capitan as sierra is no longer available, and I can't download numbers, key note and pages as they are no longer compatible. I tried open office but this didn't work either.

I have 4g ram in 2 x 2gb memory slots and it says this. Your Mac contains 2 memory slots, each of which accepts
a 1333 MHz DDR3 memory module.

I don't know what size ram I need to buy or whether it is better to buy an external memory. This is all new to me so need some help.