symbology - How to change visibility of a particular layer category in PyQGIS?

I've got one layer with its symbol MARKER, categorized by an expression in 4 colors. The colors depends on a value of a column. So from the TOC, I'm able to click and enable/disable, as they were different layers, and show one category a time.

From python script, I want to do the same, with a loop: show/hide in the map the different existing categories one each time (for printing separated the 4 jpg),

I tried to show the active layers, and categories aren't not showing as a proper layer. So, I thought that the proper layer was a group tree, but when I ask for QgsLayerTreeGroup always show no group.

How can I obtain categories for changing their visibility in the map?

root = QgsProject.instance().layerTreeRoot()
for child in root.children():
    if isinstance(child, QgsLayerTreeGroup):
       print "- group: " + child.name()
        if child.name() == "rara": #to check subgroups within test group

            print "rara"
            for subChild in child.children():
                print "child"
                if isinstance(subChild, QgsLayerTreeGroup):
                    subChild.name()
    else:
        print "no group: " + root.name()