What a beautiful dog, I have a soft spot for black labs. I think the third picture is best.
#1 - I would crop a slither off the top as it's distracting, just above where the grass ends. I would prefer his head not turned quite so much (apologies if he's a she
).
#2 - His feet are too close to the bottom of the frame when there is loads of spare room above his head. Also might have been better if you got down lower to take the shot. If you do any post processing I would darken down the background as it's quite bright.
#3 - Really like this one, the exposure looks spot on and his playful character is coming through.
#4 - Again, I think his position in the frame could be better, there is loads of space to the left and above and yet you've cropped his tail off.
These are just my thoughts, keep at it.