Jarmo Hurri wrote:

My current trout rod arsenal consists of four rods: 2wt, 3wt, 5wt and
6wt.

Yikes, only four...come on man the economy of your country is depending
upon you to do better.

I'm not experienced with the line weights I should be looking at - 7wt
to 9wt; [snip]
The rule of thumb read from the table says that a 7wt should handle a
#4 fly, while an 8wt can handle a size 1/0 fly.

A premium 7wt like the Sage XP or the Winston BIIX will handle the 1/0
fly.

However, to answer your question as to which weight to target. Use the
rule of two, since your heaviest rod is a 6 weight, you can in theory
overline it one weight to make it a de facto 7 wt. By going up two
weight classifications that would suggest you consider an 8 wt. Which
depending upon the conditions and ability over or underline as you see
fit or need.