We investigate the blue and optical rest-frame sizes (lambda similar or equal to 2300-4000 angstrom) of three compact star-forming regions in a galaxy at z = 4 strongly lensed (x30, x45, and x100) by the Hubble Frontier Field galaxy cluster A2744 using GLASS-ERS James Webb Space Telescope (JWST)/NIRISS imaging at 1.15 mu m, 1.50 mu m, and 2.0 mu m with a point-spread function less than or similar to 0.'' 1. In particular, the Balmer break is probed in detail for all multiply imaged sources of the system. With ages of a few tens of Myr, stellar masses in the range (0.7-4.0) x10(6) M (circle dot) and optical/ultraviolet effective radii spanning the interval 3 < R ( eff ) < 20 pc, such objects are currently the highest-redshift (spectroscopically confirmed) gravitationally bound young massive star clusters (YMCs), with stellar mass surface densities resembling those of local globular clusters. Optical (4000 angstrom, JWST-based) and ultraviolet (1600 angstrom, Hubble Space Telescope-based) sizes are fully compatible. The contribution to the ultraviolet underlying continuum emission (1600 angstrom) is similar to 30%, which decreases by a factor of 2 in the optical for two of the YMCs (similar to 4000 angstrom rest-frame), reflecting the young ages (