I agree with Motorman about the 400 sq in for the four TD .049 engines.
When the Tucson 1/2A multi-engine profile scale was being run for several years, there were twins, tri-motors and four-engine scale models flown in that contest. (Extra points were given if more than two engines were used.) The engines of choice were generally the Cox .049/.051 as well as the Norvel .049 and the Norvel .061. As a general rule, most found that allowing about 100 sq in per engine seemed to work out very well.
There were some twin Cox Babe Bee .049 engines used on airplanes with slightly smaller than 200 sq in.
That event was created by Mike Keville (who initiated the Vintage Stunt Championships). There were two categories: The limited event which did not allow throttles and the unlimited event which allowed throttles. Some models were entered and flown by proxy.
Of course, the 100sq in/engine is not a hard and fast guideline. Ron Duly's 5 engine He 111Z certainly did not have 500 sq in but it was powered by something less powerfull than the Cox TD .049.
Keith