Who says ones running career has to end after high school? Its ok not to hit your goal before you leave high school. Don't put so much unnecessary pressure on yourself just because its your last year. Personally I don't think its a good idea to want to put it all on the line and work “extra” hard for your senior year. Otherwise that could lead to a bad injury that could sideline you from even having a senior year cross country or track season (happened to me). One should work reasonable hard instead. As suggested just a small increase in mileage and making sure you get rest weeks when you drop the mileage down would be best.
You shouldn't be disappointed if you don't hit your goal or if you only PR by 2-3 seconds. dropping down to a 4:09 from a 4:29 does sound a little too ambitious. Maybe a sub 4:20 would be more realistic? Even that would be a great improvement and something to be really happy about. Then maybe another season later you can go for the sub 4:10. And sometimes you may have a bad season and then all of a sudden next season your times drop like crazy. Anything can happen. Just don't get injured. Consistency is better then over training. I'd say by working reasonably hard for your last year you will have a good shot at dipping under 4:20.
Just be happy with any improvement and don't worry if a season doesn't exceed self expectations. If you find that you are fit to do the kind of speed work to run a 4:09 go for it. If not then don't worry and train for a time that is faster than your previous personal best time.
You will do great