Well, that was around Alexander's time anyway. At which point the Thracians and Persians were apparently already starting to adopt the early frame-saddle from the Scythians. Given that it's a fairly helpful little item - what with increasing the comfort of both horse and rider and distributing the weight better across the mount's back, hence allowing heavier armour to be worn without wearing on the beast too much, and certainly doesn't hurt riding stability either - one would find it rather odd if the Successors hadn't picked it up in short order too.

I know the Germans refused to use saddles for really rather silly machoBS reasons, but the Hellenics AFAIK didn't have those going when it came to horse harness. Especially the more equestrian ones.

The four-horn saddle was more of a Celtic thing, wasn't it ? I recall seeing it mentioned that the way the things allowed you to "leverage" with your thighs and hips was useful for Crassus' Gallic horse at Carrhae in their rather doomed struggle with the Parthian cataphracts (who used less leaborate saddles) - since their weapon kit didn't make much of an impression, they apparently went for grappling...