Thhis is impossibly with Declining Balance 1 method. This is an accelerated depreciation method that allocates the largest portion of the cost of an asset to the early years of its useful lifetime, but if you use this method, you must enter a fixed yearly percentage, and this is not the case.
Only you can do is a User-Defined depreciation method. With a user-defined method, you can use the Depreciation Tables, where you must enter a depreciation percentage for each period (month, quarter, year or accounting period).
You need setup period on header, and after that you nid to fill lines by periods with depreciation. In this case, you must to fill period-1 and 50% period-1 depreciation; after that you must fill other periods and 4% as other period depreciation.
On FA card, you need to setup User-Defined as Depreciation Method and fill your Depreciation Table Code.
I think, this is that what you're looking for.