Multiple API Call at a time

In my current project, I need to develop a UI which contains 4 different dropdown or select controls  data of which will be populated from web api methods. I call 4 different http.get method for 4 dropdown control. But issue is that wehn I run the page, all dropdown data not populated properly. Some times, dropdown1 data bind in dropdown3. I don't understand the reason of this issue. Can any one help me.