I've been reading all the posts I can about the subject but I just need a clear answer about something...
Bottom line, are there set formulas to achieve Tier 4 plants or is it somewhat hit and miss with the mutation liquid?

Thanks in advance

To have some tier 4 plants you must buy it in the supplies or mutate it. When you have one, so you may try to progress in this tier by cross-breeding! With a lot of patience!!

The tier 4 species have a predictable breeding pattern, just like those of the other tiers. The tier 4 species can be obtained only by purchasing seeds or mutating other species, but (once you have some) they can be crossed with other species of any tier. The breeding charts in the Plant Tycoon guides clearly show the breeding patterns for all tiers.