I for many years had an imaginary threshhold on which snakes I would keep. That threshold was that I didn't want anything that wouldn't eat mice it's whole life. So I kept and bred a number of smaller species.
If you consider a corn snakes reasonably small, there are a lot of choices of mouse eating/under 4 foot snakes (dozens probably).
Here are a few that are available as captive born snakes with some regularity (some of which were already mentioned) -
Rosy Boas, Rubber Boas, Sand Boas - none get much over 3 feet (most are smaller). Small docile and generally hardy species. Males of some species stay under 2 feet.
Rat Snakes - most of the N.American Rat Snakes get too large (Black, Gray, Yellow, Everglades, Texas, Bairds) but there are many other asian species and some of those stay under 3-4 feet (a few get huge!). If you are interested, post this question on the rat snake forum and someone can give you more info about the small asian species.
Kingsnakes - while getula kings can get to 5-6 feet (or more depending on subspecies), Prairie Kings, Gray-banded Kings, Thayer's Kings, San Luis Potosi Kings, Ruthven's Kings and the two species of Mountain Kings stay under 4 feet.
Milksnakes - some stay under 3-4 feet, but in general, the ones that do are the most nervous subspecies. I would hesitate to recommend any of the small milks as a second snake.
Western Hognose Snakes - a western hognose is a great second snake. They eat mice, they are very docile and they rarely get over 2-3 feet (males are smaller). Make sure you get a captive born one that is eating mice/pinkies. Avoid the Eastern or Southern Hogs - stick with Western or Mexican Hogs (H. nasicus).
African House Snake - although females can get to around 4-5 feet, they are slender snakes (smaller than a similar sized corn) and the males rarely exceed 30 inches. Easy to keep and breed.
Children's/Spotted Pythons - these two slender Australian pythons stay under 4-5 feet and are fairly slender (not much bigger than an adult corn snake). Maybe slightly larger than what you are looking for, but worthy of consideration.
Just some suggestions to get you started.
Good luck 
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Chris Harrison
