I think the four brothers are
>! The letters **b, d, p** and **q**

The positions in the competition represent 
>! The respective positions of the letters in the alphabet

The hands and legs refer to
>! The lines extending from the circle in the lower case letters above and below the lines. The extensions upwards on the **b** and **d** are referred to as "hands" while downwards on the **p** and **q** are referred to as legs 

I can defeat my brother only as I have double power than him, this will be evident from the fact when we will grow up 10 years older." The 2nd brother hesitates and says, "Actually, he is correct. But our heights will be the same" .
>! This refers to the capitalization of B and D. B has double the bumps but both letters are the same height.

I think GirlFriends refer to
>! Adjacent vowels. **b** has **a**, **d** has **e** (the most popular) and **p** has **o** but **q** doesn't have one.