Is it safe to take 4mg of ondansetron odt with 10mg of citalopram?

Wouldn't recommend. This combination could make certain heart arrhythmias more likely because both citalopram and ondansetron can prolong the qt interval. This effect is dose-dependent for both medications.
Probably fine. These two meds are often used in the same patient. Citalopram results in more serotonin present between neurons, while ondansetron blocks a specific serotonin receptor. So if anything, they might have counteracting effects! regardless, interactions are possible between any 2 meds so check with your md first. In practice they are used at the same time quite often.
Yes. Citalopram usually does not interact with other medications, but it is a good idea to separate these 2 medications by at least 30 minutes.