Two ways. The simplest is to just use the threshold slider in the GUI and raise it up until only values of 15 remain in the overlay/threshold. Because that is also the maximum value for this dataset, no other combinations will be shown.

The second way would be to use 3dcalc to limit the values to 15 or change your original 3dcalc function to require all to be true to get a non-zero value of 1 (any values that do not meet the threshold test are zero and change the product to zero also).

3dcalc -a cond_mask2+orig -expr 'equals(a,15)' -prefix alltrue

or

3dcalc -a 'LC_groupANOVA_torland_new2+tlrc[3]' -b 'LC_groupANOVA_torland_new2+tlrc[5]' -c 'LC_groupANOVA_torland_new2+tlrc[7]' -d 'LC_groupANOVA_torland_new2+tlrc[9]' -expr 'step(a-4.2) * step(b-4.2)* step(c-4.2) * step(d-4.2)' -prefix alltrue