<ソース：sb4=sb1;のアドレス変換を初めの1回だけしたバージョン>
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
public class test {
	public static void main(String[] args) {
		
		StringBuffer sb1 = new StringBuffer("aaa");
		StringBuffer sb2 = new StringBuffer("bbb");
		StringBuffer sb3 = new StringBuffer();
		StringBuffer sb4 = new StringBuffer();
		
		sb4=sb1;
		^^^^^^^^
		
		System.out.print("---stringchange実行前---" + "\n");
		System.out.print("sb1:" + sb1 + "\n"); 
		System.out.print("sb2:" + sb2 + "\n"); 
		System.out.print("sb3:" + sb3 + "\n"); 
		System.out.print("sb4:" + sb4 + "\n");
		System.out.print("\n");
		
		stringchange(sb1);
		
		System.out.print("---stringchange実行後---" + "\n");
		System.out.print("sb1:" + sb1 + "\n"); 
		System.out.print("sb2:" + sb2 + "\n"); 
		System.out.print("sb3:" + sb3 + "\n"); 
		System.out.print("sb4:" + sb4 + "\n");
		
		
	}
	
	static void stringchange(StringBuffer sbchange) {
		sbchange.append("xxx");
	}
}

<ソース>
---stringchange実行前---
sb1:aaa
sb2:bbb
sb3:
sb4:aaa

---stringchange実行後---
sb1:aaaxxx
sb2:bbb
sb3:
sb4:aaaxxx

バッチリ(´ωﾟ)だよ