Try like this (adding the eval before your bucket command)
your base search | eval someHourField=if(someHourField >=16,">16",someHourField ) | bucket span=4 someHourField | ..rest of the search...
Try like this (adding the eval before your bucket command)
your base search | eval someHourField=if(someHourField >=16,">16",someHourField ) | bucket span=4 someHourField | ..rest of the search...