The invention relates to a compound of Formula I




or a pharmaceutically acceptable salt thereof, wherein R1, R2, R3 and R4 are as defined herein. The invention further relates to pharmaceutical compositions comprising such compounds and salts, and to methods and uses of such compounds, salts and compositions for the treatment of abnormal cell growth, including cancer, in a subject in need thereof. The invention further relates to solid forms of 8-((2-fluoro-4-(methylthio)phenyl)amino)-2-(2-hydroxyethoxy)-7-methyl-3,4-dihydro-2,7-naphthyridine-1,6(2H,7H)-dione.