This is assuming the feeders are 120/240 1 phase 3 wire?
Without knowing if there is any space limitations, your best option would probably be:
600 Amp Main fused Disconnect > 600 amp splitter install 4 x 200 amp fused disconnects off above splitter, wired from splitter to load side @ 200 amp and fuse for smaller units. (if upgrade on furnaces happen then it's only a fuse change Then come off splitter distribution panel (you can use the tap rule to keep panel size down, so it dose not need to be 600 amp. i.e. depending on load and circuits required maybe a panel with 400 or 225 amp mains will be good)
The biggest influence is probably the serving utility’s metering compartment and underground pull-section dimensions. Guessing 208V 3ø, with four 90-amp furnace branch circuits. Each furnace: (25,000W/208V/1.732)*1.25 = 87 amperes… Rough guess 600A 300V class-T fused main and a 225A-bus panel/feeder for everything else.
If I was designing this job I would use a free standing switchboard which contained a pull section, CT compartment and distribution section. The distribution section could contain the 4 breakers for the heaters and 1 or 2 breakers supplying the panels for other loads. If less then 6 breakers are installed a main disconnect would not be needed. This would also make the grounding electrode/bonding very easy since you would just need to connect 1 panel.
From reading these forums over the past couple of years I see where things like this get done very differently around the country and Canada. Apparently it is cost effective to build a service like this in some areas but I couldn’t even purchase the materials for the cost of having a switchboard built. The only way I would every build something like is if I didn’t have time to wait for the gear to be build. I can usually get something like this in 3-4 weeks (sooner for more $$$) but seldom is the equipment needed for a new building that quickly.
How would the costs change if you split the service, with a higher voltage service drop for the heaters and a 120/240V drop for your general lighting load? Is 575V available at the location; this would take the currents way down into the manageable range
Separate question, and probably not the domain of the sparky involved; why 100KW of electric heat??? We are probably talking $10K-$30K USD for the electric bill to run these heaters, unless they are run very infrequently. I would think that something fuel burning, or a heat pump system, would be more cost effective over time; though I've never thought in terms of heating large buildings.