Figure out a schedule for shower usage. You will have 4 showers. If 40 people are using them, how many showers in the peak hour? What fraction of the time is the water running?
EXAMPLE (You must do your own)
Assume that each shower takes 5 minutes, and 5 minutes to clear before the next guy turns on the water; so you have 50% water-on cycle.
If each shower takes 10 minutes you can get everyone through in 100 minutes.
Each shower uses 2.5 GPM when running and you are using 12.5 gallons per shower, so you will use 500 gallons of hot water. Add 1 gallon per person at the sinks to give you 540 gallons.
Feed water temperature (winter wells) is 55 degrees F, and shower temperature is 120 F, for a 65 F temperature rise.
Two 80-gallon heaters at 150F settings will produce 234 gallons of 120 F water, so you need to add 300 gallons of recovery in 100 minutes.
300 gallons x 65 F temperature rise x 8.34 # per gallon / 100 minutes = 1626.3 BTU per minute = 97,600 BTU per hour. That is a bit more than 28 kilowatts, or about 120 Amps. Too much for MOST instant water heaters.
That may not be enough because the heaters won't come on and run continuously. To be safe you should have a recovery rate equal to the peak demand. At 5 GPM which would result from a 50% duty cycle on 4 showers, you would need 163,000 BTUs per hour.
You could do it with a couple of 80 gallon propane-fired high-recovery gas heaters.
Do your own numbers. If you have a system where people get out of the shower area quickly so there is only a minute or two between turning the water off and the next one turning it on, then you will need a higher recovery rate.
Series connection is easy to control but may not work for you because you will want all of the firing to be working when demand is high. Therefore, you will probably want to figure out how to get them all working at the same time when necessary. Some kind of temperature control system, or one very high rate institutional heater, might be the best solution.
The heater at the link would probably work but I don't know the pipe sizes. And they are not cheap.
http://www.grainger.com/Grainger/items/6E740
Someone on the forum might be able to suggest a more economical solution.