Try Knights of the Chalice 2. It has mantis as a playable race, and they have 4 arms. You can select a maralith icon to represent it, and just pretend that it is a half=maralith. Multi-attack and off hand weapon attacks (with multiple additional arms) are both supported in that game, and can be quite strong.
I don't think that 5e even has a mechanism for using weapons in four arms, though would not surprise me if that is just me being ignorant.
