As the capabilities of high-end consumer devices increase, streaming and playback of Ultra-High Definition (UHD) is set to become commonplace. The move to these new, higher resolution, video services is one of the main factors contributing to the predicted continuation of growth in video related traffic in the Internet. This massive increases in bandwidth requirement, even when mitigated by the use of new video compression standards such as H.265, will place an ever-increasing burden on network service providers. This will be especially true in mobile environments where users have come to expect ubiquitous access to content. Consequently, delivering UHD and Full UHD (FUHD) video content is one of the key drivers for future Fifth Generation (5G) mobile networks. One often voiced, but as yet unanswered question, is whether users of mobile devices with modest screen sizes (e.g. smartphones or smaller tablet) will actually benefit from consuming the much higher bandwidth required to watch online UHD video, in terms of an improved user experience. In this paper, we use scalable H.265 encoded video streams to conduct a subjective evaluation of the impact on a user’s perception of video quality across a comprehensive range of adaptation strategies, covering each of the three adaptation domains, for UHD and FUHD video. The results of our subjective study provide insightful and useful indications of which methods of adapting UHD and FUHD streams have the least impact on user’s perceived QoE. In particular, it was observed that, in over 70% of cases, users were unable to distinguish between full HD (1080p) and UHD (4K) videos when they were unaware of which version was being shown to them. Our results from this evaluation can be used to provide adaptation rule sets that will facilitate fast, QoE aware in-network adaptation of video streams in support of realtime adaptation objectives. Undoubtedly they will also promote discussion around how network service providers manage their relationships with end users and how service level agreements might be shaped to account for what may be viewed as ‘unproductive’ use of bandwidth to deliver very marginal or imperceptible improvements in viewing experience.