I have about 15 different varieties and have grown them for 30 years. I have yet to find one that blooms before August in zone 4. In your zone they should bloom sometime in July, or even earlier.

Hi! In zone 8 I have two luna blush. They start blooming here mid to late May. June 2, and mine have been blooming over a week now.

My Luna Blush are blooming as well as a red tree hibiscus (sorry don't remember the name). I think most hardy hibiscus will bloom in mid to late May in Zone 8. Not sure though, just know about the ones I have....