The construct spirit's hit points should read "for each spell level above 4th"
I'm leaving the original answer below for historical reasons. The construct spirit stat block was given an update in the December 2021 errata of Tasha' Cauldron that clears things up:
Construct Spirit (pg. 111). In the Hit Points entry of the
Construct Spirit stat block, “above 3rd” has changed to
“above 4th.”
Confirming my original conclusion that the stat block should read:
Hit Points 40 + 15 for each spell level above 4th
As written, 55 hitpoints.
As written, the stat block is unambiguous, if a bit odd. The hit points in the statblock says:
40 + 15 for each spell level above 3rd
So casting at 4th level obviously gives 55 hit points.
There is almost certainly something misprinted here.
I am almost certain this is not intended. Every other summon something spell in Tasha's follows the traditional formula of Nth-level spell and using N+1 in the "At Higher Levels" section.
As MonkeezOnFire pointed out in comments, there are several other places in the book where the spell is referred to as a 4th level spell.
The table at the beginning of the section "Magical Miscellany" lists summon construct as 4th level, and the clockwork soul sorcerer's additional spells table list summon construct together with freedom of movement, also a 4th level spell. Further, the additional wizard spells table lists summon construct as a 4th level spell.
The preponderance of evidence points to the intention being that summon construct is a 4th level spell.
Given this, I believe that statblock should read:
Hit Points 40 + 15 for each spell level above 4th
This would give it 40 hit points when cast at 4th level.