I'm not sure how to calculate the attached equation with these multiple summations implemented into each other......

the variables a, b1, b2 in the last term include all the counters i1, i2 j1, j2:

I'm not sure how to calculate the attached equation with these multiple summations implemented into each other......

the variables a, b1, b2 in the last term include all the counters i1, i2 j1, j2:

(Answer modified since the definitions of $a,$ $b_1,$ and $b_2$ have been clarified by providing them in the question itself. I confess I did not recognize them in the question's comments, partly due to the formatting there.)
This is indeed a summation over four variables, which would be computed much the way the question describes (although I think the algorithm could be described a little simpler).
Perhaps you can use properties of the gamma function to simplify the calculation of the last couple of lines of the formula. You can at least rearrange the arguments to the gamma function so that each argument is merely a constant plus one linear term for each of the iteration variables used in that argument.