According to Ad Kongings, you coul put 10-12. I myself have 12 in a 125 gallon... I did remove a whole lot of the rockwork though.... Cut it down to some big a$$ rocks just to break the line of sight, and they're doing fine (pics following... haven't been able to take a nice one yet )
It is not the how much water your tank and sump holds. For frontosa, the floor plan is more important. Ideally, a 6 foot tank is min. to house a colony of frontosa. Some tried with 5 foot tank and the result seems to be half and half; usually work with a quad. or trio. But a 4 foot tank in my opinion is just too short.