What does "1 to 4" mean?

i've been seeing a lot of sales of uni mares in gestation with titles like "preg 1 to 4" -- what does that mean? Thanks!!
aura1600
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VIP members can use one of their Perks to have a 1 out of 4 chance of unicorns while breeding instead of the regular 1 in 5.
Roan_Lover
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