I know how you feel. My son woke at 3:30-4:30 consistently for almost a year! For the day! He's 25 months now and we are finally in a good place. My suggestions:
- try a regular / toddler bed. May not help, may only help temporarily, but try it
- have you considered a food allergy? It's when we cut dairy completely out of our son's diet that he finally started consistently sleeping well. And my doctor discounted this idea at first!! It was an ENT that suggested it.
- you could try warm milk before bed
- keep your bedtime, or possibly adjust earlier. My son was in bed by 6:30/6:45 at 19 months.
- don't get rid of the nap. A child his age needs the sleep, and he's not going to make up for it at night if he isn't already sleeping at night.
- white noise. We use a cool mist humidifier. We also have a Sony alarm clock with "nature" sounds that's tremendous. It also has a CD player so we play lullabies on it.
I second the Weissbluth book mentioned. This book saved our lives!! Our 2nd baby is a much better sleeper because we knew so much more. She's almost a year and finally is sleeping through the night. Good luck, I seriously know your pain.