Beijing, xi'an,Suzhou,Hangzhou,Shanghai in mid April. Will the weather be ok?

Yes, it can be somewhat warm and pleasant then in these places. For Beijing and Xian, the average temperature shall be 14-15C. Sometimes, it may rain a little. You can take along shirts, regular jacket and jeans.

For Suzhou, Hangzhou and Shanghai, it could be a little warmer than BJ and XA, but more rainy. Besides, the temp gap between daytime and nigh shall be large, so you may need to prepare a light sweater for outing. And simple rainy gear can be taken as well.