i found avr, avr-3, avr-4 folders
There are two places where "avr-3" and "avr-4" shows up.
One is near the top-level of the toolchain directory, and is a consequence of some sort of incompatibility between compiler versions 4.3.x and 3.4.x; I don't know the details, but avr-gcc packages (winavr, crosspack, linux installs) from those days include TWO versions of the compiler, selectable with avr-gcc-select. Arduino only uses 4.x Hopefully there are lots of symlinks involved, but I'm not sure that they survive the sort of packaging and unpackaging and re-packaging that has gone on.
The other is that in addition to the specific CPUs like AVR_ATmega328P, the avr chips are also divided into several "architectures" with minor but significant differences to compiler tools. These are unimaginatively called "avr1", "avr3.5", and so on, and affect things like libraries, startup code, and linker scripts.