I see oversized boilers on a daily basis. Perfect example today was a Vaillant 637 (37kw) when a 624 would have done it perfectly well (12 rads). The difference is that the 4 series fires on high rate where the 6 series starts on low. If you oversize a 4 series it will cause more issues than the 6 for that reason. Size it correctly and the 4 series in theory will heat up faster than the 6 though, that's the advantage. The new 4 series is totally different and they've had to make it start on low to account for idiot installers who don't know how to size a boiler correctly.