Whats the most efficient way to file 2017,18,19, and 20? have 20 ready to submit, but i have a loss-carry forward from 16 and want to get all years sorted at once.

Ideally, you should start with the oldest return and move forward from there.  That way, the loss carryforward is calculated properly and carried forward to each year.  

If you are due a refund for the 2017 tax return, then it needs to be mailed before April 15th or you may be forfeiting the refund.  

As for whether you should mail or electronically file the 2020 return, it may depend on whether you are expecting a refund or have a balance due.  If the 2020 return has a balance due, then electronic filing might be best because it is more secure and less subject to errors during processing.  But, if the 2020 return shows a refund, the refund may be delayed due to the previous year's returns not being filed.  In that case, it would probably be better to mail all the returns together to the IRS for processing.