The BS with Cooler "discovering a fifth form" was Toei and their wild, yet illogical ideas. It seems like the Cold Clan would not actually transform in the way the Saiya-jin transform, escalating and ascending in forms when needed, instead using the forms in quite the opposite fashion.
Except for the fact that Toriyama not only signed off on Cooler's 5th form but designed it himself.
Plus there's the fact Cold's in the 2nd form but doesn't bother to transform after Trunks slices and dices Freeza. These two factors make me think that Toriyama forgot how Freeza's transformations were supposed to work. Which means it could easily be considered retconned, despite the lack of outright statements saying so.
Or, you could just say that Freeza was born in his 4th form, due to his level of power, while everyone else is typically born in the 1st form. Thus his lower forms would be, for him, nothing more than a means of supressing his power. Much like if Goku had been born as an SSJ3 but stayed in base most of the time. This also leaves open room for the acquisition of new forms, such as what Cooler achieved, and not having forms beyond the 2nd as with Cold. Cold had the power to reach the 2nd form but not the power to transform beyond it.
Or, as I suggested in the Cooler thread, Cooler just figure out a way to reverse-design the forms so that, instead of creating a form with a supressed level of power, he created one with an enhanced level of power to better contain his full strength (instead of just getting big musscles like Freeza did). Of course, this does nothing to explain Cold's lack of 3rd or 4th forms. So I like the idea that Freeza was either just talking big "I'm so strong I have to transform to hold back power!" or he was simply born 4th form as an abberation.