Thanks , I read the links and they are really helpful. I increased the spacing value of flatten mapping in max also increased the lightmap resolution in UE4. Besides, I changed the lighting quality to production in order to avoid light leaks and did some experiment with the basemap.ini values. So the results are promising, not as good as v-ray renders at the moment, need more practise… I am sharing my settings down below…

And here is the result so far:
I will try out the script today Imsopov thanks…
So there are one issue left that needs a solution, as unwrap is crucial for good lighting and we need to unwrap every single object in the modelling software. How can we deal with complex objects that has a lot of polygons… I tried to unwrap a bed model in max which is nearly 200k poly, and it really took a long time, considering that i will have many furniture and whiteware in my house model, i would spend hours just for unwraping objects… Is there any shortcut for that?