When we had Buster he was 10 wks.Extremly confident and happy, but the transition did affect him, for a short while. Mainly he slept and was pretty clingy. Thinking about it I couldnt walk away for a second, never mind 4 hours.
Take comfort in knowing hes got a bond going on with your 5 year old. Buster is very close to my 5 year old daughter.