I did tract houses in CA for 10 years. I got out (thank God) when the economy crashed in 08. I did work for Pardee, DR Horton, William Lyons, Griffen, Lennar, etc. They will chew you up and spit you out. Payment schedules suck. You bill monthly, so let's say you start on Feb 1. You bill 30 days later. Then they have 30 days to pay you. Best case scenario, they will send the checks out on April 1. If your supplier releases and all your paperwork have one T that isn't crossed, they will wait the 30 days, then kick it back to you, which means you've missed the boat and have to hope your paperwork is straight when you resubmit so that you will get paid on the next cycle, 30 more days later. If your paperwork is perfect, you should get paid 60 days after you start, so you will need deep pockets to pay for your labor, ins., fuel, etc. until you get paid. Trust me, they will find a reason to not pay you on time and won't give a damn if your kids don't get to eat. The only way you will make money with the bid you gave them is if the sales team sells a ton of options. You have to price your options high. If recessed lights and ceiling fans are included in every bedroom, you will not make money, you will lose. You have to be able to sell these options to make any profit. The big builders often hire new contractors, knowing that they can't do the job for the bid given. They will squeeze you until you pop, then replace you with the next sucker.
Something sounds fishy in your OP. If you are the sole contractor for 2 companies, how is it that they only want 4 houses per month? My company was doing 4 houses (4000sq ft) per day back in the day, just for 1 builder.
Be careful.