To a large degree getting to the 3s is more about figuring out how to race than anything else, so it's hard to say whether or not it is realistic for you or not. I was in a similar boat as you for the 2008 season, I finished my 10 races at the end of 2007 and started 2008 with upgrading to cat 3 as my goal. I didn't really get anywhere for most of the season until things started to click near the end of it. I don't think I had any huge fitness leap that suddenly allowed me to upgrade.
If you aren't already placing well in the 5s then it's going to be an uphill battle in the 4s. Things will get harder before they get easier. You just have to figure out your strengths and race to them. Hopefully things will click for you and you will be able to get your points in. It doesn't take too many good results to get 20 points.
+1 Once it clicks you'll rack up points pretty quick. I strung together 6 straight top 10's this summer before I upgraded. I ended up getting 20 points at the sametime I got my tenth top 10. Get your ftp up, that'll take you farther than working on sprinting. You'll sprint better at the end if you've used fewer matches to get there. Also, think of the last 4 laps of a crit as 4 seperate races, you have to race each lap to keep position or you get swarmed.