@Loh Jane,
"Imminent" implies a sense of immediacy. It's more like "at any second" while "coming" just means sometime in the future. "Imminent" is also somewhat undefined so you generally wouldn't use imminent if you meant "at 4:00".
You sentence wasn't wrong but I doubt you would hear a native speaker saying it. If your English test is in two days, you might say it was coming. If it is in a half hour or at 4:00 you would likely just say that ("My test is at 4:00). I'd save imminent for what something is about to happen but you aren't exactly sure of the time. "The train's arrival is imminent."