4 months

Don't his legs look so much chunkier when they're bent?:

At 4 months, 2nd Brother is blowing lots of spit bubbles, grabs toys placed near his chest and chews on them (he especially likes to slime up his silky rainbow scarf), and kicks a lot while lying on his back, especially when excited.

Morning nap is 2+ hours but afternoon and evening are only about 45 minutes each (too bad since 1st Brother is in afternoon preschool and I don't have the free time I imagined). He sleeps at night from about 7:00pm-8:30am but is usually up twice to nurse. This makes me sleepy since I stay up too late expecting him to wake for that 1st feeding any minute. Must develop self-discipline soon and go to bed before midnight.

Several people have noted 2nd Brother has his own look, and I agree. Here's the same-outfit evidence with 1st Brother on the left:

2nd Brother has less forehead, smaller eyes and nose, and ears that are closer to his head I think.