i am laying crystal white tiles in my new bunglow.the grout gap is 4mm.so what grout should i use

We like to recommend that the grout colour matches the tile as far as possible. This helps to create a seamless effect.

tell me what is this

Fareham, Although from your description it appears to be grouting gone wrong thing but attached pics tell a different story. The size of floor tiles selected is not appropriate. It appears that someone may have walked over the tiles while the cement was still wet. If waterproofing was not done properly then these things may eventually snowball into a bigger problem. Looking at the pics , we would not recommend you to call the same contractor anywhere near your place again. It will be costly fix but it should be redone (at least the flooring tiles) to avoid further damages. Thanks, Nitin Cornerstone Designs, Bangalore.