For four weeks, I wouldn't worry. A battery in reasonable condition should last about 6-8 weeks.
If you are worried, and the car is parked securely, leave the bonnet popped so you can open it and put a battery charger on it after you return, if there are any problems. Or get a high capacity battery and put it in before you go.
If you know the battery capacity, and can get a tong tester to measure current drawn whilst the car is sitting still, you can crudely work out how long it will last - eg a 1000Ah battery will last 1000 hours if 1 amp is being drawn from it, although long term this will degrade the battery life.
My wife's car (a 2004 Mazda 2) started first pop yesterday after 5 weeks of not being used.
MY08 Passat 2.0 TDI Wagon
Trialling golf ball aerodynamics theory - random pattern, administered about 1550 on Christmas Day, 2011.
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