> I need to show the activation elicited by each task, the common
> areas between pairs of tasks and the common areas between all 4
> tasks - and all this on one overlay.

If I understand you correctly, the total number of possibilities you are interested in is

4 (tasks) + 6 (common areas btw any pair of tasks) = 10

out of total 16 possibilities. That is still a little overwhelming.

Maybe you can elliminate the extra 6 possibilities by adding 6 extra terms in your 3dcalac expression. For example, if you know that the final value out of the original expresion for a possibility you are NOT interested in is 7, filter with

bool(x-7)

on the output of the original 3dcalc:

3dcalc -a Anova_zscore+tlrc'[1]' -b Anova_zscore+tlrc'[3]' -c Anova_zscore+tlrc'[5]' -d Anova_zscore+tlrc'[7]' -expr 'astep(a,4.03) +2*astep(b,4.03) +4*astep(c,4.03) +8*astep(d,4.03)' -prefix My_sum_z

3dcalc -a My_sum_z+tlrc -expr 'a*bool(a-7)' -prefix My_NewSum

And do the same for all the 6 extra possibilities. Then in the output file you would only have 11 scenarios (10 + 1 for the rest) to deal with. Still break the color panel into 16 segments, but you will only have 11 colors to handle.

Does this help?

Gang