For which GTK, 3 or 4, we need which libsoup?
GTK does not depend on libsoup, and libsoup does not depend on GTK, so it does not matter.
You just need to make sure that you follow Sebastian’s advice and ensure they are never linked together. This is easier said than done currently. GNOME is hoping to transition from libsoup 2 → libsoup 3 for GNOME 42, but we’ll see how it goes.