In the old interface you had
real y[n_days, 4];
which is an array of 4-element arrays. The first dimension has n_days.
In the new interface you wrote
vector[n_days] y[4];
which is a 4-element array of vectors the size of n_days.
What you want is
vector[4] y[n_days];
or the equivalent but nicer way
array[n_days] vector[4]