Owe 4500. Can pay 1500-2000 now. Better to file an ext. or file and pay what I can?Consider interest and penalties. Currently unemployed, hope to be employed in few month

You can request an installment plan with the IRS.

You can file Form 9465 (Installment Agreement Request) with your return when you e-file. 

Please take a look at the following information on how to file the request:

https://ttlc.intuit.com/replies/4062611 

If you file your return first, you will need to go to the link below to access the form and instructions.  You will need to complete the form, following the instructions, and mail to the IRS.  The address is included in the instructions.

https://www.irs.gov/uac/about-form-9465 

Filing an extension is no help. Your tax is due April 15th regardless.
You would pay as much as possible now to minimize interest, however you
may need those funds if a job is not forthcoming.