The 4Bs can be defined as:
Bong
Bawng means 'defend' in Thai, and is the first and simplest method of defending yourself from the Tha Kru, by moving the guard so that the elbow and arms intercept your opponents attacks.
Bat
Translated as 'slap' or 'knock', Bat is one of the easiest parries to master, and is a good fallback when things get too hectic.
Bit
Bit means 'close' in Thai, and in Muay Chaiya, it means to close off the line of attack, preventing the opponent from controlling your center-line by parrying and attacking the opponent at the same time. There are three main techniques, all of which have variations and many different uses.
Beurt
One of the most natural parries from the Tha Kru guard, Beurt is deceptively simple, but combines with a range of offensive techniques to make it an essential part of any serious practitioners arsenal.
Practice
Practising the different aspects of the 4Bs is great fun alone or with a partner, and is a great way of exercising to boot. All of the techniques should be practiced from the rest guard and the Tha Kru guard, and once the basic movement is mastered, you should experiment mixing the different moves together with the other 4B movements, with attacking movements and with guards and footwork.