I'm relatively new to Plant Tycoon, and I've been reading the Forums, and seen something about Tier 4 seeds. Are those plants that are harder to get or what? Is there a way to know if you've managed to get one? Any info would be appreciated

Tier 4 species can be obtained only by purchasing extinct seeds from the supply shop or by mutation. The game will not tell you that a particular species is a Tier 4; that's something you must discover by experimentation (or by referring to breeding guides).