Have donor area less dense. Should I go for FUE as 4000 grafts needed?

4000 grafts is often sufficient for giving coverage of the frontal forelock and midscalp.  FUE gives great flexibility in harvesting this amount of grafts, but without photos or examination of your donor supply, there is no way of saying for sure you have the donor density needed for this size procedure.

This is an elective surgery.  If you consulted with a doctor, received this recommendation, and understand your risks and benefits with a good idea of realistic expectations, then go for it!  It is your choice.If you are looking for a second opinion, then you need to see another doctor in person.  The Internet may leave you with mixed messages and conflicting advice due to the lack of examination and no diagnosis.

4000 grafts is a good number to obtain a serious change with your balding on the other hand if you really have serious concerns with your donor site density then it is better to take second opinion from some other doctors too. There is nothing worse than an exhausted donor site in hair transplantation.

Without seeing your scalp and understanding your history of hair loss in detail, it's hard to say. One thing is fairly certain (provided the reason for your hair loss is genetic) - and that is that you'll get an improvement in the density of your hair.You'll want to sit down with a few surgeons to understand how thick it will be at the end result, whether you have room for more surgeries down the road, and what the back will likely look like after 4000 grafts are removed. In general, most people have 3000-7000 donor area follicular units to move in a lifetime. 

This is an art. Opinions vary. You may want to talk to 2-3 different doctors before making a decision. 

If your donor area is depleted from previous surgery or it is relatively thin, FUE may not be the best procedure for harvesting a large number of grafts. If what I am saying is true, the possibly you should consider a strip surgery which harvests only the middle of the donor area and will get the most grafts from a single surgical procedure. I am the pioneer in FUE, the first one to publish on it as I introduced it to the medical community. There are times that safety means a strip procedure when the density is poor. Get a second opinion.

Basically, if you need a bigger session with limited donor hair then it is best to consider FUT. In  Follicular Unit Transplantation (FUT), the hair for relocation is obtained by surgically excising a strip of hair bearing skin from the donor region of the scalp. The skin edges of this wound are then sutured closed. Follicular Unit Extraction (FUE) is a revival of the old punch technique obtaining donor hair with a small circular cutting instrument. These punches are used to harvest follicular units one at a time from the donor area. Each harvest or extraction creates a small circular scar. The difference is with the strip you get a continuous linear scar, whereas with punches you get discontinuous, round circular scars. You get a better Yield with FUT technique because one can see the follicles when excising the strip whereas in FUE it's a blind extraction from the skin. Due to the blind extraction, there may be damage to the hair follicle and also collateral damage to the surrounding hairs. Therefore, it is likely that when hair is transplanted to the recipient area in case of FUE technique some of the hairs be damaged and therefore either not growin or grow thinner.  So with a limited donor hair available, and for higher number of grafts needed to cover a large recipient area, to get the best yield with least collateral damage FUT harvesting technique is recommended.