It has certainly been noted that the divide between minion and standard is rather large. There's always room in there for something in between. It would be easy enough to build a stat block with half normal hit points and maybe one simple power and possibly a trait. They'd be useful as monsters that won't just fall down if you toss auto damage at them, but can still be ganked with a single solid focused hit. Going with 1/3 or 1/4 normal hit points might even be best, but you'll have to experiment a bit. Maybe make them worth 50% XP of a standard.
This is better than using lower level monsters as those will both prove pretty much ineffective, are more complicated, and probably still have a bit more hit points that you want.
My basic approach going forward in my next campaigns is to halve all monster hit points, but use more elites and solos (which I have previously avoided); converting some standard monsters in published adventures to elites as appropriate. Encounters can be run a couple levels higher to maintain the threat level. XP from non-minions will thus be more generous; I have found it rather stingy, esp compared to minion XP. And I may reduce XP for lower level monsters, say 1/2 XP if more than 3 levels below party level.