I'm assuming your a male who is concerned with his butt? I also assume that you do not want to get in top, all around BB'ing shape, but do wish to add some muscular size to the arms and chest (is the reunion is pool party?). I assume you also want to impress folks that 20 years has been kind to you. (have not been to my 20th yet, but have been told that the size of your wallet and status of your profession will cause more attention that the fitness of your body) Well all of this can be more than possible with-in a 4.5 months. And you do not have to make every training session torture or every cardio workout a endless marathon. Improved muscle size and fat loss can happen rather quickly, if your training and recovery time are in balance. It does not take years, for most people.
With regards to cardio: You might consider the Tabata protocol, which can challenge the aerobic and anaerobic abilities. Giving benefits to both with regards to superior fat lose and muscle recovery, both at the same time. You are training the CNS, in so many words, with this very intense form of exercise.. The base of this is 20 seconds on intense effort and 10 seconds of rest followed by another 20 second effort, etc, etc, etc. Each 20 second effort and 10 second rest period is a set. Usually four to six sets a Tabata workout. Eight sets seems to be the max in any workout. And usually 2 to 3 times a week. Spinning offers the best avenue but stationary bikes, reclines, jumping rope, rowing machines,etc can be good. Even punching a heavy bag for 20 seconds can work. Twenty second sprints (or hard running) can be applied to this system. Actually just about anything that require short, intense effort seems to work.
Lifting programs should also be short and to the point, with exercises directed to compound movements, show the possible best results. Hold the workouts to 3 a week for muscle gains. I have seen many men and women make very good results with a upper body workout (chest, back, delts, arms) on Monday and friday and hitting the lower body on wednsday (legs, calves, abs). The following week that is reversed with monday and friday being lower body work. And wednsday being upper body work. Full body workouts offer another way for max gains in a short period of time. But remember again, short, brief and to the point workouts.
Try controling the carbs , with the target view of getting 100 grams a day. If results are still not to your liking ,than reduce the carbs down to between 40 and 60. Do not go below the 40 gram a day mark. Please avoid using Vasopro, too much caffeine or anything else of like matter. These are for the extreme BB'ing who's system can handle these stronger supplements. Not designed for the average 38 year old guy who just wants to get into decent, and impressive, shape. Good Luck.