ECN Forum Forums Code Related Discussion NEC & other Code issues min ampacity of feeder for sub panel 4 ckts

On a sub panel installation in a residential garage of 4 two wire branch circuits I say that the min feeder ampacity should be 60A. Based on the fact that the disconnecting means must be rated at 60 A by code. My co-worker says that it is based on the computed load and could be 40A or 50A. Please advise with code reference. Thanks