How to detect concrete or brick behind plasterboard
You do not have to anchor into masonry for this load. Drywall with proper anchors will easily hold a 4 kg load. That first type of anchor you show is an old and outmoded type. There are several new types that are much better, and the 2nd picture shows one of those.

You might have to cut off the screws that come with it, if they contact the hard surface behind the drywall. No pre-drilling is required to install the 2nd type. Get a large Phillips screwdriver and force it into the drywall to the first threads then screw it in snug, but do not over-tighten. If the sharp end contacts the hard surface behind the drywall, then back it out, cut off enough of the sharp end so that it no longer contacts, reinsert, and screw in.