Hi, what is the temperature like in Beijing come April ?

For the weather, you can check Beijing Weather(https://www.travelchinaguide.com/climate/beijing.htm) , better find the newest weather forecast before coming. It would be warm, some times a little windy in April. But it would be cold in early April, if you mean that.

There is train to Great Wall, subway to Forbidden City, but no train to Ming Tomb, you need to take bus or taxi. From Wangfujing Street you can walk to Forbidden City, it is very close. Enjoy your time!