I don't want to have to reenter all the medical receipts for that latter 8 months of 2017, entered last year, which I will be claiming along with the first 4 months of 2018. How do I get them into my 2018 tax return I'm working on?
Unfortunately there is no way to transfer these medical expenses forward to your 2018 tax return.
If you do not need to (or chose not to) use all your medical in a particular tax year, you must "remove the entries", and then reenter them the following year. If you already "claimed them" whether they resulted in a deduction or not, you'll need to file an adjustment because those amounts will appear on 2 different returns, and CRA will not allow it.
Seems strange after all these years, no functionality is built into the program to carryforward the unused expenses from the medical expense worksheet. It's such a waste of time entering various medical expenses in the first place and then needing to "erase" them if unusable due to the 3% limitation.
Here is the list of items that can be transferred to next year.