Is it possible that the server is underpowered could be a cause?
Remember that WebDirect is generated in the same context that a server-side script is run in. So while it may work for you on a desktop client, the context is different and may behave differently depending on how your relationships are formed.
That said, what's more probable is that the rendering engine for WebDirect just can't reliably go 4 levels deep when it draws portal objects.
My suggestion, if you absolutely need it, would be to cache the data in a single related table, and load those rows into your table instead. Personally I avoid portals entirely in my web based solutions and just use List <-> Form --> List style navigation, only ever using list and form views.
Wish I could limit things to no portals.
It occurred today but only two levels deep.
the are simple single predicate equi joins between number fields