I want to run 4 lamps of 1000 watt on diesel engine. I want to check that is 6 kw output generator is sufficient for same. I want full calculation for this, losses etc
You want to connect a 4 kw load to a 6 kw generator. As long as the 6 kw (6,000 watts) rating of the generator is accurate it should power the lamps fine. You're only using 66% of the generator capacity. There aren't any significant losses to worry about and no point in doing any calculations. They were done for you in the kw rating of the generator. Just don't use any extension cords that are too small for the lights.
