I hate to say this but...
4 weeks is not enough training time for a crit. Training for a crit is NOT like cramming for a final. To see real gains on a bike, you need to start out with a base training time period, followed by endurance training, followed by strength training, followed by interval training, followed by power training. The minimum amount of time I could recommend for this type of training is 8 weeks, and even then, your gains will be minimal.
What I would advise is perhaps doing interval rides- rides where you work on mixing aerobic with anaerobic timed intervals (ie 10 minute warm up bringing heart rate to 75% gradually, then 2 minutes at 85% max heart rate, 2 minutes at 75% recovery for 45 minutes, then 30 sec- 1 min at 90% max heart rate and 2 minutes at 75% recovery for another 15 to 20 minutes, then cool down on the bike for 10 minutes, taking heart rate to 60%). Repeat this type of session for 2 days in a row,then take a rest day. Follow that rest day with an endurance ride for 3- 5 hours going at 80%, and go no more than 5 beats less than 80% if you need a break from riding at 80%, and go no higher than 5 beats above 80% if your heart rate goes a little higher than the 80%. Then take a cooldown with heart rate going to 60% by the first 2 minutes into your 10 minute cooldown. Do this for 2 more days, then take a recovery. Then repeat the interval session for 2 more days, then take a recovery day. Repeat the 2 day endurance session, then take a recovery day. For your last four days, you're going to work on power training- Take the warmup for 15 minutes, gradually bringing heart rate to 80%, then alternate between 90% heart rate for 30 seconds and recovery to 80% for one minute. Do this ride for 45 minutes, then take a 15 minute cooldown, gradually taking heart rate to 60%. The next day, take a recovery, and I don't care if you feel like you don't need it, just take it. The day after your recovery, repeat the power ride with alternating between the 90% hr maxand 80% hr max like I outlined with the warm up and cooldown. Next day, take a recovery. On the 17th day, ride your century. If you can get 2 days off after that century, do so.
Repeat process for the second part of your 4 week training period. That actually gives you 38 days instead of 28 days, but figure out how you can get those extra 10 days in.
If you don't use a monitor, get one. You can get a low end, continuous read monitor like that Polar A2 or a low end Timex monitor (I prefer the timex because it's digital, so the reading is instantaneous, whereas the analog monitors take time to convert the heart rate from the strap before giving the reading, so it's sluggish when you're doing interval rides). You can get a good Timex from Walmart cheap, or try www.heartmonitors.com
and get one there- they have good customer service. If you don't know your heart rate numbers, either get to a performance testing lab in your area, or do a search under my name for the 2 X 20 Anaerobic Threshold test so you can self test. Let me tell you right now- if you can easily reach 90% and feel like it's hard, but easily achieveable, your numbers are OFF. A 90% effort should feel like you want to die and can't be maintained for more than a short period of time (30 seconds for amaeturs and 1- 3 minutes for pros). I took a forum member recently and put him through the 2 X 20 Anaerobic test, then at the end, we tried to get him to 90%. He got within 5 beats of his 90%, then had to stop. I thought that was very good for a first time effort... I know he'd never seen that number on his monitor before, but that's what 90% effort is- it's a number you don't see very often, if not at all. So don't fool yourself if you haven't been tested and have trained before and think you can use some old school math formula to find your heart rate numbers and use them. Most likely, they will just make you slower, since more often than not, I find people training under their true numbers.
This will probably not get you up to the level of the folks who have trained properly for these races, but I guess it'll take you a bit closer, which is better than where you are right now, right?