With 4 & 3 gallon kettles, your max boiling capacity is about 5.75 to 6 gallons. Boiloff loss will probably be 0.75 to 1.25 gallons over the course of an hour, leaving you with about 4.25-4.75 gallons to drink, including trub and transfer loss. I guess you can top off with 1-3 liters of water at the end to get 5 even gallons. You'll have to use both kettles for the boil (straddle each kettle over two gas burners).
For the mash, get yourself a 5 gallon nylon mesh paint strainer bag from Home Depot. Put about 4-7 lbs. total crushed base and specialty malts in the bag, secure with a rubber band, and mash in 147-149 F water for one hour in the smaller kettle with a tight fitting lid. Use 1.5 liters water per lb. of grain for the partial mash. Prod with a plastic paddle every once in awhile to make sure the grains are completely wet and also test that the temperature is constant. When complete, you'll want to collect and measure the grain wort. Make sure that when you combine it with the main volume of water, that you're starting at 5.75-6 gallons total.
While your grain is mashing, you'll want to heat your main volume of water to 170 F in the larger kettle. When the mash is complete, let your grain bag drain and then dip it over and over again in the main kettle volume to rinse the residual sugars. Let drain once more, and place over a large bowl. Mix the mash wort with the water of the main kettle volume. Bring both kettles containing an even mix of this low OG wort to a boil. Once boiling, add half of your DME and your bittering hops. Start the timer at 60 minutes and proceed with the rest of your recipe. Add the rest of the DME at flameout.
Best to know your water chemistry too. I always put a little gypsum and calcium chloride in my cold brewing water because I need to up my calcium, sulfate, and chloride levels. These minerals dissolve better in cold water. The added sulfate especially helps to offer more of that hop bite.
Recipe to follow.
