What to do if my car was hit 4 days ago and the driver said he would pay without getting insurance involved, however he just now told me that he’s not paying?

You should file a claim for your property damage with the at-fault driver's insurance carrier.  If he doesn't have insurance and if you have uninsured motorist coverage on your auto insurance policy, you can file your claim with your insurance company.  If the other driver does not have insurance and you don't have uninsured motorist coverage, you will need to sue the other driver for negligence.  Your lawsuit should name the at-fault driver and registered owner of the vehicle if the registered owner is someone other than the driver.  Prior to filing the lawsuit, with regard to what I said above, if the registered owner has insurance, you should file your claim with the registered owner's insurance carrier if the registered owner is someone other than the at-fault driver.

If there is no insurance and you don't have uninsured motorist coverage, file your lawsuit for negligence as mentioned above.  Your damages (the amount you are seeking to recover in your lawauit) would be the cost of repairs to your car.  You will need to mitigate (minimize) damages by selecting a repair shop whose charges are comparable to other auto repair shops in the area.  If you were to select the most expensive repair shop you could find, your damages would be reduced accordingly.  If you need a rental car while your car is being repaired, you will also need to mitigate damages by selecting a rental with reasonable rates.  If you rented the most expensive car you could find, your damages would be reduced accordingly.  Depending on the amount you are seeking to recover in your lawsuit, you might be able to file your case in Small Claims Court.  In addition to the cost of repairs to your vehicle and rental car cost if applicable, your damages should also include court costs.  Court costs would be the court filing fee and process server fee.  If you get a judgment against the opposing party, it would be advisable to also obtain a wage garnishment to enforce the judgment given this individual's track record of refusing to pay.