The max is 2 surrounds.  

Only two surround speakers is supported.

Suggestions as to how to hookup the two other era speakers to play at the same time?

You can group the two extra speakers with the Arc setup, but you will likely experience a slight audio delay from the grouped speakers when playing TV audio. Streaming music will be in sync though.

In all honesty, use them someplace else.  If you group them, not only will they be delayed, they are only going to be in stereo, and they will be playing all channels, so the surround and height channels will be mixed in with the fronts, messing up the soundstage and ruining any directional cues.  More speakers do not mean better sound, so stick with the optimum configuration.