Updating a table with a single UPDATE statement vs several UPDATE statements
When update executed once it's guaranteed that all rows that satisfy conditions will be updated (or none in case of error). 4 separate updates may update different rows (something may change between updates, and you're using MyIsam, so transactions won't help;  in addition, your update depends on non-deterministic NOW()  ). 

Another point is that each UPDATE locks set of rows , so 4 updates will lock rows 4 times even if the same rows are affected(in case of MyISAM)