A Stack Overflow with a Difference
This level is pretty straight forward. Just sit down and understand what the code is doing. Your shellcode will require a setuid(LEVEL4_UID) since bash drops effective privileges. You could alternatively write a quick setuid(geteuid()) wrapper around bash.
NOTE: ctors/dtors might no longer be writable, although this level is compiled with -Wl,-z,norelro. Lookup some information about this e.g. here