MDX Intersection - how to?
You just need to add an MDX where clause. Look at the Basic MDX syntax.

In that where you need to specify the SET to slice on. Have a look at Using Sets in MDX Queries

SELECT 
NON EMPTY { [Measures].[Order Ref Product Count] } ON COLUMNS, 
NON EMPTY { ([Product].[Product ID].[Product ID].ALLMEMBERS ) } ON ROWS 
FROM [Sales]
WHERE {[Product].[Product ID].[3],[Product].[Product ID].[4]}


In my personal opinion it's no use trying to relate MDX queries to SQL queries. Both are an entirely different beast.

You need to stop thinking in tables and joins, but in tuples and sets and members and measures.

One of the best resources in my opinion to get started is the Stairway to MDX series.