Divide physical ethernet port to two virtual

Recently I've got a bit odd idea and did some googling but found nothing (Closest is this super user question but I think my idea is not the same). The idea is, as I think, relatively simple and kinda lies on the surface so I'm surprised there's nothing about it out there. The question is: is it possible to divide one physical 8-terminal ethernet port into 2 by 4? The idea came to my mind because I've got one Raspberry Pi with one ethernet port and I thought could I use it as a kind of poor hardware firewall? I've got a server at home and the cable that goes from my ISP is 4-pinned, so I thought to make a cable that would have 3 heads, with one in the middle, so that one would go from my ISP and connect to the Raspberry Pi and by the same connector, but with other terminals connect to my router. Maybe it would require writing/patching an ethernet driver or something, but I'm just curious