According to the IFRS 3,
Acquisition date is the date at which the acquirer started to control the assets of the acquiree. In this scenario, Jan 1, 2015, the date of stock purchase, is the acquisition date.
Value of the asset
The value asset must be moved from the carrying value to the fair value. Let us consider the fair value of the asset is $8000, so the value of must be moved from the book value [10,000-4000=6000 (10,000/5 x 2years) ]to the fair value ($8000).
The useful life of the asset remains the same if no indication for change in life mentioned by the acquirer ie 3 years in the scenario. So the next year depreciation will be calculated as ; $8000/3= $2666.67.