This generalization is TOTAL BS... especially on the Verbatim you've mentioned and it is entirely dependent on the writer, the software and the settings!
Maybe I generalized a little but my intention was not to spread bullshit in this case. It's because on 99% of burners people have that's how it works. I'm talking from a good amount of experience.
Most burners do better at 4x than at 8x with the DVDs I mentioned and even if you have a very good professional burner (wich I have among the others) that perfectly burns DVD games even at 8x, there would not be any contraindication in burning at 4x. Again, at least on Verbatim DVD-R.
You could open a thread about this matter, where everyone can post their tests so to do some statistics if you want. Perhaps you'll prove me wrong.