I saw the Lobsterman and I was thinking "CR 4?! Maybe for the whole party." I then looked at the rest of them with that thought in mind. Yes. You did great! These things were MEANT to be handled by a whole party. I remember playing the game it took me at least 3 of my men to fight one Lobsterman. The players in my current campaign will now be able to face actual challenges! My only problem is I will have to get them (the players) more able to function in combat (they struggled in a battle against three Butter Golems fought one at a time)(There were only 3 players at the time but they are all level 7 and the Butter Golems are CR 3 each). But anyways, great job.
Although, remembering the competence level of the party, might wanna check the numbers. Maybe a little higher. But I love it!