Is it possible that we could see a 4-core Macbook 12" with the release of Intel's Cannonlake architecture?
Entirely possible
Entirely not possible maybe afterlake's
Cofeelake will bring 15W quad core chips, which can be slightly downclocked to have 4.5W TDP. Coffelake is 14 nm architecture. Cannonlake will be after Coffelake and with 10 nm architecture, which can easily have quad core CPUs on 4.5W TDP.