It will be particularly under CR'ed if used as a boss monster. As I understand a lvl 4 party should be able to take down a CR 4 creature with no real problem 925% of resources expended), and really be challenged by CR 6ish. A CR 4 monster will often be used as a boss encounter for lvl 2ish parties this monster doesn't seem suitable for that.

I seems like a creature better designed to be use in multiples against higher level parties (2 of these against a lvl 6 party seems like a fair fight) rather than as a boss monster or straight 1 vs party monster.