Catalysis of the reaction: 4-CDP-2-C-methyl-D-erythritol 2-phosphate = 2-C-methyl-D-erythritol 2,4-cyclic diphosphate + CMP.
2-phospho-4-(cytidine 5\'-diphospho)-2-C-methyl-D-erythritol CMP-lyase (cyclizing)