What will you be washing that 2 guns running only 4gpm will be productive? Car lots so you can have 2 guys rinsing them down instead of one guy? I would not do any commercial work with only 4gpm as you will be stuck with small surface cleaners like 16" or 18" for the small water flow. You might be ok with house washing with the smaller flow but more gpm's you can work faster.
I would stick with the 8gpm machine and don't split the flow.
More than likely you can buy a used rig or build a rig with the same components for a lot less than a new rig with a fancy name. You can think up a better, cooler name to call your trailer once you save a ton of money building it yourself, probably save enough money to get a second 8gpm rig so you can have 2 guys working about twice as fast as 2 guys with only 4gpm.
There are rigs for sale right now on the bbs's, check them out. Also look on Craigslist.com and ebay for some great deals.