I have followed several derivations to obtain the formula for tunnelling probability in a finite square potential well. the standard result that is actually an approximation to a more complex result is; I get the derivation of this result shown in many standard texts. what I do not get is the term before the exponential always approximates to the number 4. I am not interested in verifying this with actual values for V, E. I want to know algebraically why the term is always approximately equal to 4 for any values of V, E. it is supposed to be obvious and probably is, eludes me atm. authors often make a further approximation and ignore the entire term so they are always out by a factor of 4 on an already approximate answer. when dealing with such small numbers is leaving the 4 out that pedagogically sound - that's a seprate question.