Android findFragmentByTag切换选项卡上的奇怪行为

我在ActionBar中切换了5个片段：

@Override
public void onTabSelected(Tab tab, FragmentTransaction ft) 
{
    int tag = (Integer) tab.getTag();
    fragmentTransaction = fragmentManager.beginTransaction();

    switch(tag)
    {
    case 0:
        FragmentBase fragBase = new FragmentBase ();
        fragmentTransaction.replace(R.id.frame_order,fragBase,"FragmentBase");
        fragmentTransaction.addToBackStack(null);
        break;

    case 1:
        Fragment1 frag1 = new Fragment1();
        fragmentTransaction.replace(R.id.frame_order, frag1, "Fragment1");
        fragmentTransaction.addToBackStack(null);
        break;

    case 2:
        Fragment2 frag2 = new Fragment2();
        fragmentTransaction.replace(R.id.frame_order, frag2, "Fragment2");
        fragmentTransaction.addToBackStack(null);
        break;

    case 3:
        Fragment3 frag3 = new Fragment3();
        fragmentTransaction.replace(R.id.frame_order, frag3, "Fragment3");
        fragmentTransaction.addToBackStack(null);
        break;

    case 4:
        Fragment4 frag4 = new Fragment4();
        fragmentTransaction.replace(R.id.frame_order, frag4, "Fragment4");
        fragmentTransaction.addToBackStack(null);
        break;

    }
    fragmentTransaction.commit();
}

然后在函数中，我想从片段4中获取一个值：

Fragment4 frag4 = (Fragment4)getSupportFragmentManager().findFragmentByTag("Fragment4");
    String stfrag4 = frag4.getFrag4Value();

问题是，当我在标签4上时，这是我的Fragment4内容页，

findFragmentByTag

的返回值总是空的，但当我切换到另一个标签，然后找到该片段时，它返回的是真正的片段而不是空的。 为什么会这样？我怎样才能解决这个问题呢？提前感谢。

进行测试

改变

int tag = (Integer) tab.getTag();

借

int tag = getActionBar().getSelectedNavigationIndex();