Exactly, Joe Riel! Thank you!
And now, one more level of complication: I need to make the vars list itself be of indefinite lengths. Plus, I have a lot of R's to compute, so I'll have to put your code into its own loop. More specifically, I am computing c[i,j,k,l] for all i,j,k,l in terms of (using my recursion, as linear combinations of) just those of the form c[0,j',0,l'] and c[i',0,k',0] where i',j',k',l' run from 0 to about 2*i,2*j,k,l. Hence, the number of variables c[i',0,k',0] and c[0,j',0,l'] needed increases with i,j,k,l. I need to compute a sufficient number of c[i,j,k,l], as I mentioned earlier, about 0<=i,j,k,l<=5, to visualize the pattern and hopefully guess a general formula.