npregress series: How can I fit an additively seperable model with more than one variable per additive function?

Hey,

Im trying to fit a semiparametric model with npregress series.
The model I want to fit should look like this: y=bx + g(z1,z2) + h(z3,z4). I no that the nointeract() option can do that, however I am not sure how to achieve this specific form.

Kind regards
Helene