What Kind of Laser to Use on Cobblestoned Skin from Hair Transplant?

...is a bit early to start using laser.  I'd wait minimum of 6 months then follow up with the surgeon who performed the procedure.  Co2 on a light setting may improve the scars down the road.

It is difficult to treat cobblestone scar after hair transplant surgery with satisfactory results.  You need to see a doctor for an exam before considering options.  Many patients with lasers/ fraxel / derm abrasions have tried but often find little improvement.  Each case is different.

It is too early to intervene at four months. What appears to be cobblestoning might resolve before the one-year mark. Revisit the matter if it persists beyond the one-year mark.

Surface irregularities in the skin following hair transplant are a common occurrence and will often improve in time without treatment.  Since you are four months post-surgery, I would not recommend laser treatment at this time. With any type of skin resurfacing treatment, you run the risk of damaging the transplanted hair follicles.  If the condition hasn't resolved after a year, you should see the plastic surgeon who performed your transplant for treatment recommendations.  Good luck to you.  

It takes about 8 months for all new hair to mature after a hair transplant.

Treating the areas with lasers will only increase the chance of new hair being damaged potentially.

Once you reach the 8 month mark re-evaluate condition.

bumps on the skin after a hair transplant usually reflect the graft placement, the graft size and the overall techniques that were used. A laser will have no impact on these bumps

Since it is only 4 months since your transplant, I would suggest waiting another 6 months to see if this concern still exists.