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

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.

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.

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.