findViewById(R.id.btn4).setOnClickListener(
    new Button.OnClickListener() {
     public void onClick(View v) {
      
      //imageview에 열기
      Intent intent = new Intent(getApplicationContext(), Web.class);
      intent.putExtra("target", "http://www.ifull.co.kr/m_index.html");
      startActivity(intent);
      
      //익스플로러로 바로 열기
      //url = "http://www.ifull.co.kr/m_index.html";
      //startActivity( new Intent( Intent.ACTION_VIEW, Uri.parse(url)));
      //finish();

     }
  }); 

=== xml ===

    <LinearLayout
        android:id="@+id/l_btn_view"
        android:layout_width="fill_parent"
        android:layout_height="fill_parent"
        android:orientation="horizontal"
        android:layout_gravity="bottom|center_horizontal">

        <ImageButton
            android:id="@+id/btn4"
            android:layout_width="fill_parent"
            android:layout_height="fill_parent"
            android:layout_marginBottom="5dp"
            android:layout_marginLeft="5dp"
            android:layout_marginRight="5dp"
            android:layout_marginTop="5dp"
            android:layout_weight="1"
            android:background="@drawable/btn_searchlist"
            android:padding="0sp"
            android:scaleType="centerCrop" />

    </LinearLayout>

=== btn_searchlist.xml ===

<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android" >
    <item android:state_pressed="true" android:drawable="@drawable/a8bt2_2"/>
    <item android:drawable="@drawable/a8bt2"/>
</selector>