The D4 D'ama Cruiser is actually newer than the D6 Cruiser, but by virtue of the Klingon design nomenclature is numbered as an earlier D-type model cruiser. This is by virtue of the fact that the D6 and D4 design projects overlapped, with the D4 being a hold over project that had been moribund from the 2180s.
The D'ama Cruiser was an early attempt at building an uprated cruiser hull. In this regard it is similar to the D6 Cruiser and other ships of the direct D-type lineage (of which the D7 Battlecruiser would later be a part). The D'ama shares many striking design and aesthetic similarities with the D6 Cruiser. Overall the D6 Cruiser was thought to be the more successful of the two, but the D'ama Cruiser did have its merits within the scope of the fleet. It was the first ship to be equipped with more advanced sensor systems and a heavier load of disruptors compared to other ships of the time period. However it did so at the loss of any torpedo weaponry, a staple of most Klingon warships of the last century. Where D'ama Cruiser had been tailor made to serve more in the role of a heavy cruiser, the final production model fell far short of being worthy of this name resulting in a change of nomenclature.
A threat in the way of an unknown, exterior alien threat along one of the frontier borders of the Klingon Empire in 2217 lead to a hurried production run of the D'ama Cruiser, much to the worry of Federation and Romulan forces that believed that it was a prelude to an invasion by the Klingon Empire. The D'ama served proudly in this role and earned itself a name within the fleet. In the next decade the D'ama and D6 both would contribute real-world combat data that would lead to the production of the next in the D-series of cruisers: the D7 Battlecruiser.