Enzyme of the plastid non-mevalonate pathway for isoprenoid biosynthesis that converts 4-diphosphocytidyl-2C- methyl-D-erythritol 2-phosphate into 2C-methyl-D-erythritol 2,4- cyclodiphosphate and CMP. Also converts 4-diphosphocytidyl-2C- methyl-D-erythritol into 2C-methyl-D-erythritol 3,4-cyclophosphate and CMP. Is essential for chloroplast development.