Although Ryan’s answer is technically correct it should be pointed out that Id’s are unique. There is only ever one id of the same name on a page so there is no need to concatenate them with a class (unless you are using ids on multiple pages and changing their appearance via a local context).
#tab4{}
That’s all you need.
Note though that ids have more specificity than classes so the styles in the id will always win out.
To avoid specificity issues you could do something like this.