Hi!
I also have a 4 month old ds and yes, I think it's definitely normal (to a point). You know that feeling you have after taking a midday nap? Kinda groggy, mildly lethargic, etc? I think that's what happens (at least it is with my ds).
If it was true tiredness it would continue until I put him down for another nap, and yet it doesn't. I have found that a much better indication is fussiness, not yawning. A friend on here from the 4-6month board said a wise thing recently: that if you change their scenery and they are still fussy it's probably that they're tired, but if they stop fussing when their environment changes, it's something else.
Now, that's all IF they don't continue...if he yawns for much of his A time (starting at 30 min or so) it very well could be that he is needing more S.
Hope I've helped...a little.
