90 degree down angled USB connector for iphone 4s
Have you considered a 90 degree USB adapter instead of a specialized dock/30-pin cable?  I'm using this right angle adapter with my Alpine head unit and a 10' lightning cable and have no issues with my iPhone 6 or 7 plus.  

Right angle USB 3.0 adapter

If you absolutely need it to come out of the head unit and straight down then you should be able to use one of these with similar results.

L Shape USB 3.0 adapter

The "accessory not supported" messages usually are a result of an aftermarket cable that isn't manufactured within the guidelines of the Apple MFi program.  This pertains more to the lightning or dock connector end rather than the USB connection end of the cable.  Because of that you should be able to use just about any USB adapter to meet your needs.

Hope that helps!