David swims the 100-meter freestyle in 48 seconds.  He swims the 100-meter backstroke 4 seconds slower than he swims the 100-meter freestyle, he swims the 100-meter butterfly 3 seconds slower than he swims the 100-meter backstroke, and he swims the 100-meter breaststroke 2 seconds slower than he swims the 100-meter butterfly.  What is the combined length of time, in seconds, it would take for him to swim all four 100-meter events (the freestyle, the backstroke, the butterfly, and the breaststroke)?
Please reason step by step, and put your final answer within \boxed{}.
user