Which sub to purchase : mini or 4 ?

What type of music do you like? What equaliser settings do you like? Are the Eras lacking in bass at present? 
 

The Era 100s are pretty small so using the mini with them is going to be well matched acoustically.

That may not be what you are looking for and if you want a lot more low end then the full Sub would be a better choice. At worst you turn it down a bit rather than finding it inadequate for your ears.

In that sized room I’d have been tempted to the Era-300s or Fives. Many folks find the Fives adequate with no Sub, the 300 seems to benefit more from having one.

The key (asked above) is what you want.

For me a Pair of 300s in a 10x10 foot office with a full Sub is a good fit. Maybe a bit more than needed but I wanted the best Atmos experience I could get, so the 300s, and since I had a spare full Sub it was the cheap option. They pair well and neither are pressed to more than fill the room.

I have a Pair of 100s in an open plan room, 15x20 feet and they do a decent job of background level music. They, or that room wouldn’t be my first choice for loud and bass heavy music but for the size and cost they are great sounding. I haven’t felt the need for a Sub in there and the spouse isn’t interested in giving up the space one would need.