It's not as if there was no warning, but when it happened no one had time to properly doubt its existence, sudden as it was. The signs had been there for a long time, yet the senses that interpreted them were veiled in such a way that they were perceived only as shredded, non-coherent anomalies, nothing more.
Then, one day, something vast was suddenly torn ever so slightly at it's edge, causing it to give way and thus letting the visage of the other side seep through. Again, it was not fully recognised by the four silhouettes present at that point, at that time, but somehow they all knew what it implied.
Gradually, they realised that as they focused and reached through the veil together they simultaneously erased all borders between their four perceived gestalts and flowed through the breach as one.
The new unity was given a path, a pace and finally, it needed a name. A name that honoured the collective point of origin and recognised the complete ignorance of its eventual destination. The path became music, the pace steady and driving, and the name finally given to describe all this was Primate X.
It was either that or "Monkeys with suits, ties and hats."
The pinnacle of something.
A shining beacon of whatever.
Too late for warnings now.
Just join the experiment, you know you want to.