War bonus on clash of clans

Basics:

War bonus is determined by the base that you attack and your town hall level. When you select an enemy base, the bonus will be visible in the rectangle that pops up (the one with the options to scout and attack). The higher up the base, the bigger the war loot will be if you are victorious in your attack.

To answer your question, yes, if you attack a town hall below yours, you get a 'penalty'. I just tested a few numbers and asked some fellow friends what they see as war bonus and I have the numbers confirmed to be just like the normal loot penalty figures:

Here's a table:

Town Hall level difference  |    Percentage of War 
                            |   bonus loot available
----------------------------+------------------------
Same or higher level        |          100%
1 level lower               |           90%
2 levels lower              |           50%
3 levels lower              |           25%
4 or more levels lower      |            5%