Hello and welcome to JustAnswer.
I'm sorry to hear about your situation and hope I can help.
If you were fired for an on the job injury, you would have a claim against your employer for discriminating against you for filing a worker's compensation claim.
I would suggest that you consult with a local attorney as soon as possible to discuss your cause of action.
Additionally, your employer cannot withold your final paycheck by requiring you to sign a severance agreement prior to receiving your check.
If they do so, you would be entitled to the amount due to you, plus a day's wages per day you were required to wait to receive the wages due to you.
You should file a wage claim with the Department of Industrial Relations, Dvision of Labor Standards Enfrocment regarding your unpaid wages, which you can do online here:
If they have failed to pay taxes on your employemnt to any state or federal entity, they are required to pay those taxes plus penalties not you.
If you fill out another W4 you should list the number of withholdings you actually have, and not just 0 as requested by your employer.