I worked as a substitute in the school from 10/19 to when COVID closed school 3/2020. Can I collect unemployment?

And you didn't find out the answer to your question before now because?  You've had plenty of time to file a claim and to have been either approved or denied.  So do it.

That depends on the exact laws of your state.  Unemployment laws do vary slightly from state to state.

Two issues here - your time of employment may be too short AND you might have waited too long to file a claim.  

You can always apply for unemployment.  Read over the information VERY CAREFULLY and if you find some reason listed that disqualifies you - then be sure to cancel your application.  In some cases, benefits will be paid and then revoked later leaving you to pay back what was given because you should have paid attention to the information and not finished the application if you found a reason not to qualify in the information they give you.  They don't always catch who doesn't qualify until after they start paying you - then they cancel you and tell you to pay back what had been issued.

Not in my State - 6 months of employment are required.

Yes you can.  They may cut you off as of the end of the school year.

4 months, and you are just now asking that?