Adaptive bitrate (ABR) streaming is one enabling technology for video streaming over modern throughput-varying communication networks. A widely used ABR streaming method is to adapt the video bitrate to channel throughput by dynamically changing the video resolution. Since videos have different rate-quality performances at different resolutions, such ABR strategy can achieve better rate-quality trade-off than single resolution ABR streaming. The key problem for resolution switched ABR is to work out the bitrate appropriate at each resolution. In this paper, we investigate optimal strategies to estimate this bitrate using both quantitative and subjective quality assessment. We use the design of 2K and 4K bitrates as an example of the performance of this strategy. We introduce strategies for selecting an appropriate corpus for subjective assessment and find that at this high resolution there is good agreement between quantitative and subjective analysis.