I don't think it's a deformity, though given 2 very similar heifers, I'll go for the one with exactly 4 teats. Last year one of the heifers I kept comes from a line which nearly always has great udders, however this one has an extra teat right beside a real one, and I was thinking of putting a band on it. Meanwhile, one of the heifers that calved this year has 6 teats, and they all work, though the hind pair don't have much in them ( I guess that's where the expression "Left sucking hind teat" comes from).
My criteria on a mature cow is the bottom of the udder shouldn't be lower than the navel, and the teats should be like my thumb... about 2 1/2" long, 1" around... that makes it easy for the calf to find them.... also, they should point downward even when the bag is full... if they are too close to the leg, that makes it harder for the calf to find them as well... Last but not least, there should be something in them!
I'm starting to select my replacement heifers for exactly 4 teats, and I think after a couple generations that should make for some better looking cows.
Though she might have some lacks, this cow has the type of udder I like and makes lots of milk. when she's not milking, she looks like a heifer... no udder whatsoever
And her 15 year old mother... a little sag is acceptable