What will happen if my ETS date is 4 1/2 months into a deployment?

You are probably NOT going on deployment. Get with your CoC for clarity

You will be placed on RearD and given the time required to transition out unless you are placed under a stop loss. If a stop loss is put in place your enlistment will be extended to cover the deployment.

You’ll be offered to extend to go through the deployment plus 90 days to demobilize. If you don’t do that, it’s up to your commander whether you will deploy and how much leave you’ll take. You are required to return and have at least 90 days to transition. You are not required to be given transition leave