High Entropy Alloys (HEAs) represent a promising class of materials known for their exceptional properties, often unattainable in conventional alloys. These properties include high hardness, strength, ductility, and resistance to thermal effects, radiation, and corrosion. Among HEAs, a particularly notable group is the new generation of high temperature materials [1] - Refractory High Entropy Alloys (RHEAs) - which have attracted significant attention due to their ability to maintain high strength even at temperatures up to 1600°C. In our previous research on the CuMoTaWV RHEA [2], we demonstrated that its average hardness and nanopillar compressive strength were approximately 20% higher than those of other RHEA thin films reported in the literature. These promising results motivated us to further investigate the deformation mechanisms in this alloy. Therefore, this study focuses on the deformation behavior of MoTaWV-based HEAs with the addition of a face-centered cubic (FCC) stabilizer-Cu-resulting in the composition Cux(MoTaWV)1-x. Our objective was to determine the optimal Cu content that enhances both the mechanical properties and high-temperature performance of the alloy. To achieve the research objective, behaviour under various loading conditions was investigated using nanoindentation and compression of micropillars in-situ Scanning Electron Microscope. Experimental data were correlated with captured images and videos, which extended analysis. Microstructural changes induced by compressive stresses were investigated using Transmission Electron microscopy with TKD analysis to enhance knowledge of the behaviour of the material after loading. Finally, this work evaluated the influence of various effects on the mechanics of RHEAs, including BCC-FCC interfacial deformation and addition of FCC former to BCC-based material.