Only 4 additional uniforms possible??

I have written a simple shader to render simple bricks.
It works, but ...

It seems to me that shaderFrog has a limit on the number of uniforms (besides the predefined ones),
and that limit seems to be 4!
Is that true?

You can find my shader here: https://shaderfrog.com/app/view/5292

It works only because I am hard-coding some of the values of the uniforms at the beginning of main.
But no matter how I order the uniforms at the beginning of the source, I only ever see 4 uniforms in the uniform editor below the source!

Is that ShaderFrog or am I making a mistake?