My dear, dear, sweet, adorable, cuddly, chubby-cheeked son~
Forty minutes is nowhere near an acceptable length for an afternoon nap for a four-month-old, especially when you seem to think your morning nap should only be half that length. You can keep protesting, but I will remain firm on this one. You will learn to take an afternoon nap of at least an two hours (three hours or longer would be ideal). When you outgrow the swing, I will find another means of forcing you to fall asleep.
I love you, and will do just about anything for you... Except let you win this battle.