I usually do it the other way around. Re-dosing doesn't have the full effect compared to taking it all at once (50 mg) would. You lose a little bit because the opioid receptor gets desensitized temporarily after the first dose, so it doesn't "see" all of the second dose. The main reason why you would want to split up the dosage is if you have concerns that you are taking too much and might OD. My personal experience is that it's best to wait at least one hour before redosing. An hour is usually enough for you to know if you need more or not. The half life of perc and hydro is fairly short so 4 hours is probably too long to wait before taking more.