I think you are doing something wrong. I think you're missing a step...
Say you're in 4th at 2700 rpm. Now you wanna jump to 2nd. You push in the clutch, pop it out of gear, pop the clutch, rev up to where 2nd would be for your current speed, for me it's probably around 4800rpm or so. Push back in the clutch, put it in 2nd then release the clutch again, feeding it the proper gas. You shouldn't feel ANY jerk, if you do, you did it wrong.
Basically, when you're trying to mash it into 2nd from 4th, your tranny isn't gaining the rpm it needs to get into second. When it won't go into gear, it may be a sign that you're going too fast to get into that gear properly anyway... be careful.
If you haven't been driving stick too long, then I would not get into double-clutching just yet. Get a feel for your car, know where each gear's RPM is compared to the current gear and RPM. It'll take time, but in the end, your car will thank you.