I am sure is a security venerability is discovered in 4.2 they will patch it.
Actually once they stop support they're pretty much done putting out updates for that device, even security holes.
When the jailbreakme.com exploit came out they updated and patched the exploit with 4.0.2 on all iphones but the iphone 2G that was left behind on 3.1.3
Cant blame them though.