I qualify to FMLA according to the federal guidelines (employer has 50+ employees, etc.). I also have sick leave benefits and accumulated vacation days, which add up to more than 4 months. I have two questions:
1) current supervisor has been very abusive as he has given me tons of extra work that don't correspond to my job description, even so, I have been fullfilling his requirements and done the job. I have a dr.'s note where my dr. authoirzes me to be out of work until further notice, however, my supervisor says that he will not authorize any leave until I come back to the office and get the job done. What are my rights?
2) I'd like to take time off back to back. Is it possible to take those 4 accumulated months (paid) and follow with a 12 week FMLA leave?

Any guidance will be greatly appreciated as I am in a great predicament considering that I have always felt committed to every job I held.

thanks

STD/PTO/ FMLA are not generally mutually exclusive.

You can be out of work on fmla and still be paid your pto or disability while you are out.
You would not traditionally take the 4 weeks of pto after fmla. If your employer has a generous leave policy, then you could potentially take all the time they allow.
Employer must grant fmla when applicable, they are not obligated to grant any other or additional leave. (Except certain states).

So if your asking if you could take 16 weeks of leave, you only have
protection for 12. The rest depends on you employer's policies.

Your HR department should really be the one to address your leave.
It seems you are already out of work, so you need to start the fmla process yesterday!
If you are out on approved fmla, you cant be forced to work.

Sorry, I thought it was 4 weeks pto, but it is 4 mos. So it would be 28 weeks total not 16 as I said.

The employer can (and almost certainly will) require you to use any paid leave you have available as part of your 12 weeks of leave under FMLA. Since you have four months (16 weeks) of paid leave available to you, it's possible that the employer may allow you to take 16 weeks TOTAL off and use up all your available leave. It doesn't have to, however.

Your supervisor doesn't get to authorize your FMLA leave. If you need to be out on FMLA-qualifying leave now, I'd suggest contacting your HR department for assistance with your supervisor.

It is EXTREMELY unlikely that your employer will allow you to take 4 months of paid leave and then follow up with 12 weeks of unpaid FMLA leave for a total of 28 weeks, nor does any law require them to.

You can certainly ask, but I wouldn't be betting the rent on having it approved.