I have 2 TB of iCloud. Can this be increased to 4 TB?

Not currently. Apple does not offer a larger iCloud plan beyond 2TB at this time.

If you seriously have over 1.8TB taken up, you may want to consider a cleanup, maybe download older stuff you don't use as frequently and transfer it onto an external drive.

Apple doesn't offer a plan larger than 2TB; you'd need to either use separate 2TB plans on different Apple IDs, or find a non-Apple cloud storage service.

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