Can't get 4th level drop-down menu to work

[deutz]

Hi and thanks in advance.

I have a 3 tier menu that works fine but when I try to add a 4th level, I cannot get the 4th level sub menu items to align to the right of the 3rd level sub menu parent. Instead they appear on the 3rd level menu inline with the parent sub menu item. The 4th level sub menu items are the ones with a year followed by 'Part A', 'Part B' etc. I have attached a text file with the HTML and CSS.

Thanks kindly
Deutz