Make sure they are getting sufficient light. "growing like crazy" can sometimes be that they are merely stretching for more light. You want stout, stocky seedlings, Especialy heading lettuce like iceberg because they can't support the head on the seed stem below seed leaves. I recommend supplemental lighting with fluorescent light.
If you think they are healthy seedlings, it is possible to separate 4-day old seedlings by floating them soil mix and all in water. If they could be better, I'd say start again with only 2 (or 3) seeds per cell and clip off the extra one (small pointed embroidery scissors, nail/cuticle scissors work well for this purpose) so there is only one per cell.
i think it might be too early to start cucumbers and cantaloupe. You want to start them about 2-3 weeks before last frost and wait about 2 weeks after last frost to plant them out if you are in a hurry. Most cucumbers arne cantaloupe mature fast enough that you can sow them directly in the ground about a week to 2 weeks after last frost and still have good harvest. (unless you live in a very short growing season area -- you'll want to choose fast maturing varieties in that case.)
On the other hand, you may be able to sow snow peas and spinach directly in the ground outside already.