When the Klingons decided to sell the Romulan Empire some of their older D6 cruisers, many in the High Command balked at allowing a still-viable warship to be sold off to a potential threat. This resulted in the Klingons searching through their fleet to find the most 'suitable' candidates for export. They wanted to get ships that were begining to show their age enough that they would soon begin to need extensive refits; yet the ships had to be sturdy enough to stave off any suspisions that they were trying to cheat the Romulans.

This would take quite a while, and so the Klingons devised a plan to appease the Romulan need for results. They cleaned up 18 old D4 heavy cruisers from the fleet's training squadrons and offered them to the Romulans along with 21 of the new E3 escorts. The Romulans, desperate for hulls with which to defend their space, reluctantly agreed to the terms and accepted the ships as well as the delay in the delivery of the D6's.

The D4's were in reasonably good shape for ships their age, but they did not warrent the kind of attention that either the more sturdy Old-Series ships or the newer Klingon designs that were slowly coming in. As a result, the D4's did not recieve ph-1's when the Romulans re-armed them. The shortage of beam power was more than made up for, however, by the number of marines and transporters that each ship carried. The standard tactic for one of the YKR's was to knock down a ship's shields with her two plasma-G's and use hit-and-run raids to damage the target... or simply to capture th opposing ship outright.

The success of this ship in the role of escort and heavy police ship ensured it a lasting place in the Romulan fleet, and it was kept in service long enough to recieve the B-refit, which finally saw the YKR getting it's boom phasers upgraded to ph-1's. The Romulans even requested 40 more of the old ships, but the Klingons had totally phased them out of servive. Instead, they simply sent the plans for manufacturing the D4 hulls to the Romulan government. By the time the plants were brought on line, the first of the New-Series ships were coming on line. This bumped the YKR from front line service, but many were still produced as police ships and trainers due to their simplicity and versitility.