I am new to the game been reading the rule book can some one tell me what is the armor value of the defense strenght of a squad of 4 when it is being attack please some one help me.

A squad of 4 (infantry) has no inherent "defense"/armor rating. A squad will take casualties when an attacker rolls a 5 or 6. A squad only has a "defense" rating if it has cover - such as certain terrain (e.g. woods), or structure (e.g. entrenchment, building).

Vehicles have defense ratings, but these are worked out the same way as cover.

infantry squads have no armor ratings, only vehicles.

What infantry squads do get are what is called cover bonuses.