I'm a nonresident of ME with a rental property there. It was rented for 4 months. I reside in CA. Do I need to file a ME return if I had a net rental loss for the year?

As a nonresident of Maine, if your only activity in the state was a loss, you are not required to file a return. Per the 1040ME instructions:

"Who must file?... Generally, if you are a nonresident or a “Safe Harbor” resident who has income from Maine sources resulting in a Maine income tax liability, you must file a Maine income tax return."

https://www.maine.gov/revenue/forms/1040/2018/18_1040me_book_dwnld_rev_3.6.19.pdf

Thus, if you have no other ME income which would result in a tax liability, you are not required to file a return in the state. If you have other income sourced from ME, you may need to file a return.