Excerpt from the AZA care manual for JSC's:
"It is recommended that the aquarium exhibit have a minimum of 1,500 gallons (5,680 liters) or 1.2 square meters (12–14 square feet) per animal (e.g., a group size of six crabs can be housed in a 9,000 gallon (34,000 liter) aquarium exhibit). Inter-individual distances required by ex situ JSCs are not known with certainty. It is generally assumed that more space is better, and at a minimum, there should be enough room for each crab to stand normally, without touching another crab."
MBA's tank is 220 square feet, and using the higher recommended value stated for four JSC we get 56 square feet. However it seems in this case we're dealing with large crabs than average; regardless the room I'm currently in is about the same dimensions as the tank and it seems pretty large visualizing 4 large crabs in here. I guess we'll have to see once the exhibit opens.
Link to the full publicly available JSC ACM if anyone wants to peruse it.
https://assets.speakcdn.com/assets/2332/japanese_spider_crab_care_manual.pdf