Oddly enough, 3-4 is the weak point of Eberron adventures.
The following Dungeon adventures have notes for adjusting to levels 3-4.
#113 – “The Queen with Burning Eyes” by James Wyatt (1-5)
#117 – “Fallen Angel” by Keith Baker (1-5)
#133 – "Chimes at Midnight" by Nicholas Logue (3-6)
#136 – "Tensions Rising" by Ryan Smalley (2-6)
#143 – "Riding the Rail" by Christopher Wissel (3-5)
#147 – "The Aundairian Job" by Craig Shackleton (3-5)
Of these, only "Tensions Rising" is written for those levels (level 4). It doesn't look too bad, but I haven't run it.
I recommend "Chimes at Midnight" highly, but adjusting will be a lot of work (removing class levels from quite a few characters). I also recommend "Fallen Angel."
One of the Eberron trilogy adventures was written for those levels (Grasp of the Emerald Claw perhaps?), but I don't personally recommend them.