Extract recipes can start with 1-6 gallons, it's just a matter of adjusting the bittering hops. Generally, a full boil for an extract 5 gallon batch only needs to allow for an hour's boil-off. That's about a gallon.
Since you only have 4 gallons, your final gravity will be higher than the target. Extract, PM, AG, I just top off to 5.25 gallons in the fermenter.
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