Question about a file called content.ggpk

4 GB sounds oddly small. The full content.ggpk should be about 12 GB, at least on the standalone client. I don't know about the Steam version of PoE specifically, but usually Steam doesn't keep duplicates of files. The file being in Steam's downloading directory also seems odd. Perhaps it's a leftover from an interrupted update? You can safely try deleting it; if it was required after all, Steam will just download it again.

download poe from pathofexile.com instead of stream so you dont have to trouble yourself with those extra files and save space