ukulele as starter bass???

I think any cheap ukulele will suffice. The key consideration is the action and how easy is it for a 4-year-old to hit a fretted note and be satisfied so as not to lose interest (or you, a pile of $$ if it all gets too hard)
Basic piano training to establish finger strength and dexterity is probably more important at this age IMHO
We're into phase 2 of this process now and have our daughter playing a 25" scale custom bass - which is basically a strat neck fret spacing & scale.
 
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At four?

If your kids are anything like mine, four is so young that getting ANY instrument into their little paws is great. I wouldn’t get hung up on the number of strings, nor the type of instrument. If they’re willing to play it, go with it.

I agree, just getting into music in general is a blessing and there's a lot to learn regardless of instrument. Learning to feel the beat, to recognize pitch and intervals, a lot of finger coordination. Or read music if going that route.

One of my 1st grade classmates could play the violin. She moved away a few years later, wonder how she turned out.
 
Go for the Rondo then :).
The point is not kala or rondo, the point is the tuning and the lower register than that on a ukulele.

Remember we're talking about a 4 year old. The chances he'd even care or be able to tell the difference in tuning between a regular uke and a bass uke is slim at best. Also, his interest in "playing bass" may not be more than just a momentary fascination with an unfamiliar object. I still say get a cheap regular uke and see if he's into playing music. If not, at least its not $200-400 wasted on a bass uke.