I've used regular Bosch, the Platinum 2s, and Platinum 4s...out of all of them, I feel the Plat 2s ran the best. The engine was the most responsive under load, and pretty much under all conditions.
From what I've heard, the Plat 4s put too much strain on the e30's electrical system. Mostly that they can't carry enough current to produce a good enough spark. Personally, I don't have factual evidence to substanciate that, but I can tell you that the engine doesn't feel as good as it does with the singles, and no tnearly as good as the 2s. I'm currently running 2s.