Back when the PlayStation 3 was first released, many believed that Sony were never going to catch up to the amount of units the Xbox 360 was selling. However, with the help of a new slimline variant of the PS3 with larger storage space and a cheaper price point, Sony managed to claw it's way to a position that put it neck and neck with Microsoft's Xbox 360.
This time round, Sony have gained the clear sales advantage over Microsoft and the Xbox One, but just like before, anything could happen to give the losing console the chance to wing it into first place.
If the Xbox One manages to get a price drop and a selection of strong console exclusives, sales for the console could easily start to catch up with that of the PlayStation 4, and if that happens, Sony are reportedly prepared to drop out a new PlayStation 4 slim variant, which like the PS3 slim, will most likely come with a slim line frame, a cheaper price point and a larger hard drive.
It's likely a 'slim' version of the PlayStation 4 will release at some stage, but it also makes sense that a release will only kick in when sales start to slow down, and Xbox One sales start to pick up.
Right now, the PlayStation 4 is drastically beating the Xbox One in terms of units sold, and this huge difference in sales numbers isn't likely to change any time soon. Perhaps we'll see a PS4 slim in a few years.