If you use a 4 gallon bottle with DIY, do you use double the recipe and get equal CO2 to using 2, 2 gallon bottles? I have a 55 and everyone says I should use at least 2 2 gallon bottles and since I have a 4 gallon soda bottle available, can I just treat the 4 gallon bottle as 2 2 gallons?
You can. One reason people use multiple CO2 bottles is so they can better maintain an even output of CO2. For example, they can alternate the bottles they change every week so that the flow never gets really low like you would get at the end of a single bottle.