VENTILATION for a gas boiler and 2 hot water heaters?

A 100,000 BTU gas boiler for a heating system and two 80 gallon hot water heaters are located in a basement. This is not a question about the exhaust vents into the chimney. It is a question about adequate air flow for the combustion. Right now there is an opening into an alley with bars and screening to let the air in. It is rather large as at one time the location housed a gigantic coal furnace. The opening is a problem because of rain, cold and pests. I would like to seal it and replace with one or two 4" PVC pipes leading to the outside attached to two 4" dryer vents installed in reverse, i.e. to let air in rather than out. The pipes would be about 4 to 5 feet long each with a loop at the top to keep the rain out. Do you think two 4" vents would be adequate to ventilate the systems and provide sufficient air flow?