Like it or not, Google has a pretty proven track in this area... I put the chances at the Nexus 4 officially getting L to be around 20%. As for zorak950's comment about another Nexus device, until just a few hours ago I thought the Nexus line of phones was completely dead seeings how all stock had mysteriously disappeared from the Play Store in recent weeks, but now that we see a
Yellow Nexus 5 is likely to be coming, plus some of the leaked pictures of L are shown running on Nexus 4 device, so who knows what the future holds... it's not like Google hasn't surprised us with things in the past.
Even if we don't officially get Android L, the developers of ROMs for the Nexus 4 will pretty much ensure we get Android L and beyond for some time, in "stock AOSP" and customized versions. There is actually pretty heavy discussion on this in the XDA developer forums, it is fully expected to have a relatively stock Android L ROM port fully working on the Nexus 4 within a couple days, it seems they already have a bootable version but it buggy... and yes, Dalvik support is gone completely, ART is the only working runtime.