Did s/he go previously & have a toilet back up scare? Has s/he never tried? As dr kim states, break it up into useful stages + rewards.
Entering the bathroom stage1/ disrobing 2/ sitting 3/ sitting 1min 4/ making poo stage 5.A 4yo can tell you what they value, so rewards should be focused on their wants & easy to deliver. Some are slower than others but be patient and supportive. New is stressful.