I had 4500 grafts and now the doctor recommend that I can do another 4000. Is it safe?

Read my post which discusses the limits to FUE which you may be pushing here: https://baldingblog.com/2017/07/21/many-fue-grafts-many-fues-grafts-one-know-safe-limits/.

Over-harvesting the donor area will create balding in the back of your head. In my experience, you may have already exceeded the limit. In the photo, the text is over the areas of differing densities to call your attention to it. If you go ahead with another 4,000 grafts, the doctor will make money and you will be bald in the back of your head.  Not a good deal even if your doctor did it for free.