Q:

Why we use fourth finger of left hand to start b major?

I am just curious about this thing because recently i have started learning music. 

A:

Scale fingering for piano is largely based on using the thumb as a pivot point on the most appropriate white notes.
So B major starts with white key, then has two black keys, then a white key, then three black black keys, and so on. After you start the scale, your fingering should always cause the thumb to land on a white key in order to "pivot" and continue the scale. So, if you play a multiple-octave B major scale, you will be playing B with the thumb of the left hand. But, in the upwards direction, the pivot happens right after this note. So, when starting off the scale, there's really no reason to use the thumb on that note, since you have enough fingers to get yourself to the next pivot point of E on the thumb.

A:

Thumbs are generally shorter than the accompanying fingers. On pianos, the black keys are further away from the player. So, it makes sense to use fingers on black keys whenever possible, and use the thumbs for white keys, which are closer and easier to reach.By starting the B maj. scale, L.H. with your ring finger, the next two notes played will both be black (C# and D#), leaving the next note, E, to be played using your left thumb, which is just in the right place.
When you play R.H. of Bb maj., you'll probably use the same trick, starting with middle finger, and putting your thumb on the next note, C.Then two fingers on  Db and Eb, leaving a thumb handy for the next white key, F.

