Can I file the 4868 extension even If I don't pay tax?
Yes. But remember that form 4868 extends your time to file your individual tax return and NOT the time to pay taxes on your income. IRS would charge penalties and interest for the tax due. This could be avoided if you have reasonable cause for not paying. You are accepted to have reasonable cause for not paying if 90% of your tax due are paid (within the deadline for filing) through withholdings, estimated tax payments, payments made with form 4868 , and, the remaining 10% is paid while filing the return.