Mender does not modify the partition sizes on already deployed devices. Meaning that if you have devices out in the field that have a 4GB data part size, that will remain unchanged on those devices.
Any changes you make right now will only affect “new devices” that you provision with that configuration. And since you are going from bigger to smaller sizes that should not be a problem, because the Mender Artifact (possibly ext4 payload) size will be smaller and should fit on the devices that had the following definition,
MENDER_STORAGE_TOTAL_SIZE_MB = “14784”
MENDER_DATA_PART_SIZE_MB = “4096”
What you will not be able to do is to deploy “older” Mender Artifacts that where created using above configuration to the new configuration.