In main-sequence, low-mass (<1.3 Solar mass) stars, rotation and magnetic activity decrease over time, with the spin-down thought to be due to angular momentum lost through magnetized winds. This magnetic activity heats the stellar atmospheres, leading to the production of Hα, ultraviolet (UV), and X-ray radiation, each originating from different atmospheric layers. Measurements of these emissions and of rotation periods (Prot) for low-mass stars therefore help constrain the strength of the underlying magnetic fields and their dependence on Prot. We present a multi-wavelength study of stars in four open clusters; Group-X, Coma Ber, Praesepe, and the Hyades, with ages between ≈300 Myr and ≈700 Myr. We use Hα equivalent width measurements, GALEX UV photometry, and X-ray detections to characterize the magnetic activity of stars, and we combine them with Prot measurements from TESS and ZTF light curves to examine the dependence of activity on rotation. We also compare the three activity indicators against one another for the same stars to understand how magnetic heating may differ at different atmospheric layers. These results provide essential insight into the relative efficiency of magnetic heating of the stars' atmospheres, and they help to characterize stellar radiation environments and the habitability of orbiting planets around low-mass stars.