There are several aspects to this.  First, fuel consumption will vary with the load, and some generators will perform better over 50% load.  The manufacturer will have information on the fuel consumption at the various loading of the generator.  Usually, 75% will provide the best fuel efficiency, and 50% or below may supply the worst, while 100% will tend to be higher than 75%, and in some cases still better than 50%.

The other thing to consider is what kind of load your generator is maintaining.  Mechanical loads (motors, compressors, fans, etc.) have a variable because the spinning motion is not equal.  By comparison, solid state devices (UPS, Transformers, IT equipment) have more static loads, their variance is due to their load, not a constant spinning motion.  So to adjust for that a "power factor" is usually applied to the generator capacity to indicate what the risk factor should be in such loads.  This 80% of total is due to harmonics (impedance) that is accumulated on the line due to the spinning motion.  If your load is purely static, then you don't need to apply this power factor.   (But note this is rarely the case, there are some circumstances that may make this possible).  However I note in your question 4.2MW rated, and 3.8 "firm capacity" (that's not an industry term), but what I believe is you have a .9 power factor applied to the generator for usable capacity.  So your maximum in this case is 3.8MW.  The generator manufacturer documentation should provide clarity regarding this point.

maximum efficiency get at approximate 80% load condition

There are several aspects to this.  First, fuel consumption will vary with the load, and some generators will perform better over 50% load.  The manufacturer will have information on the fuel consumption at the various loading of the generator.  Usually, 75% will provide the best fuel efficiency, and 50% or below may supply the worst, while 100% will tend to be higher than 75%, and in some cases still better than 50%.

The other thing to consider is what kind of load your generator is maintaining.  Mechanical loads (motors, compressors, fans, etc.) have a variable because the spinning motion is not equal.  By comparison, solid state devices (UPS, Transformers, IT equipment) have more static loads, their variance is due to their load, not a constant spinning motion.  So to adjust for that a "power factor" is usually applied to the generator capacity to indicate what the risk factor should be in such loads.  This 80% of total is due to harmonics (impedance) that is accumulated on the line due to the spinning motion.  If your load is purely static, then you don't need to apply this power factor.   (But note this is rarely the case, there are some circumstances that may make this possible).  However I note in your question 4.2MW rated, and 3.8 "firm capacity" (that's not an industry term), but what I believe is you have a .9 power factor applied to the generator for usable capacity.  So your maximum in this case is 3.8MW.  The generator manufacturer documentation should provide clarity regarding this point.

Thank you very much for your detailed explanation@Scott Payton

Thank you very much....Prof.@ Sanjay R. Vyas

Based on load-efficiency curves, maximum generator efficiency is achived at about 85% of load condition. 

Thank you very much for your information. If possible please share the link or source of load-efficiency curves@Mohanad Alfellag

Gas turbine efficency depends on air density (temperature) also. Both factors are to be considered. Manufacturers provide the necessary information. Firm capacity might be referred to the operation at maximum ambient temperature.

Thank you very much@Giulio Santagostino

Giulio is right.  My assumption was that it was not above 1,000 meters.  If it is over that, then density will start to play a factor as well.