confusing where clause in SQL Server

I want to have a simple condition in my where clause, on one of the columns like:

When @PostTypeid is null then where should be PostTypeid in (4,5) and if @PostTypeid is not null then where clause should be PostTypeId = @PostTypeId

I am trying to use case for this purpose but I think I am getting confused with the syntax. Here is what I have come up with so far:

 (PostTypeid = case(@PostTypeId) when null then in (4,18) else @PostTypeId end)

The error is when I use in, I dont think that's a valid syntax. What is the correct way to do this? thanks. I Tried shifting the braces to several other places but in vain.

Thanks

Something like:

WHERE
    (@PostTypeID IS NULL AND PostTypeId IN (4, 5)) OR
    (@PostTypeId IS NOT NULL AND PostTypeID = @PostTypeID)

perhaps?

You cant use = and in operators in one statement

EDIT:

(PostTypeid = case(@PostTypeId) when null then in (4,18) else @PostTypeId end)

if the @PostTypeId is null then it will goes like this.

(PostTypeid = in (4,8)) 

and this is syntax error.

case(@PostTypeID) when IS NULL then PostyTypeId in (4,18) else PostTypeId = PostTypeId

this case should return single result.