How to use TEXTSPLIT() on a range?

Suppose I have a 6-row x 1-col array -- in A1:A6, say -- where each cell contains a sequence of four letters separated by commas. For example:

A1="a,b,c,d"
A2="e,f,g,h"
...
A6="u,v,w,x"

I want to produce from that a 6-row x 4-col dynamic array where each row consists of what you'd get if you ran TEXTSPLIT(<cell>,",") on each cell in the original array.

Why does this not work:

=BYROW(A1:A6, LAMBDA(row, TEXTSPLIT(row,",")))

And how should it be done?