(at around 1h 25 mins) All the Critters from earlier movies have the infamous green-goo instead of red blood, but at the climax of the movie when Ethan kicks the frozen Critter into the wall you can mostly only see red frozen blood parts fall off of the Critter - no green-goo.
The eggs Charlie find in the beginning are supposedly eggs from Critters 3, but the Critters in Critters 4 are totally different - they are smaller and have different appearances.
(at around 9 mins) Although significant dialog and on-screen dating indicate the setting is the year 2045, Ethan's hand-made calendar visibly shows August 2342.
Charlie mentions the eggs "take six months to incubate". On Critters 2, this is untrue when 3 boxes of eggs multiplied at the meat factory into an enlarged critter ball.
(at around 37 mins) Obvious puppets used in first killing scene. When the attacked character runs and slams his hand towards walls and objects you can see it's just a Critter-puppet attached to his hand. Later on when the character is dead you can see the Critters move and blink their eyes normally.
Boom mic or camera team briefly visible at the ending, reflecting on Charlie's sunglasses and at the red button he plans on pushing.