It's not your fault. Your brother is a big boy and capable of making his own decisions, and he should've been more careful than he was.
I'm going to assume that the stiff sentence would have more to do with him probably being on probation for something else already than anything else, right? I can't imagine someone getting four years for a bag of 'shrooms but then again I'm not familiar with every state's laws, especially not regarding hallucinogens. With all of that being said,
IF that is the case and he was on probation (or hell, even if he wasn't but already had a record for something else), he should've realized that he was putting himself in danger by using drugs. He should've learned from his last run-in with the police and avoided putting himself in a position where he could get arrested again.
People have free will. If he didn't find something like this through you, I'm willing to bet he would've found something through another person.
Perhaps you should take this with a grain of salt and decide to no longer provide your brother with any connections knowing that he's not very careful regarding his own actions. You can sit around and be mad at--or even blame--yourself by saying he wouldn't have gotten arrested otherwise but come on, you know that's not true. Stupid is as stupid does and he made a stupid choice. If he hadn't done it through you, he would've made a stupid choice someplace else. The best you can hope for is that he learns from this and doesn't end up serving any real time in prison.
I wish your brother the best of luck and I hope he quits being an idiot

Once again, it's not your fault... We have free will at the end of the day and so on and so on.