You won’t find many drives that burn CD-R at 4x, especially not modern drives with modern media. The same goes for modern 16x DVD media, although a few [very few] drives can handle underspeeding to 4x with good results.
I’m personally suggesting:
Samsung external 18x or 20x drives [good CD and DVD writing quality, generally]
Pioneer external 18x or 20x drives [good CD writing; generally really good DVD writing]. The exception is their 20x drive with LightScribe, the DVR-113/DVR-1910, which is an Asus rebadge and hasn’t been overly praised for its abilities [but is still decent].
If you go internal, the above, plus Optiarc 20x burners with model numbers beginning with AD-72xx [very good writing all around]. I’m not sure how many Optiarc drives may be found as external drives. But you might be able to buy an internal and an enclosure to make it external, which is not that difficult.
Sony is currently rebadging Samsung drives, or using the drives from their joint venture with NEC [which is Optiarc, the same from above], so you won’t go bad there, either internal or external. In the Americas at least, the drives are rebadged Samsungs. I believe Europe mainly gets the Optiarcs.
As far as I know, Mad Dog is still rebadging Samsung drives for both internal and external drives.
Plextor’s new drives may be anything now, since they stopped making their own drives. But the PX-810 internal SATA drive is a Pioneer rebadge [albeit, an old one].
Others may chime in on what they feel is best. Each of the above has its strengths and weaknesses.