The Hipshot D-Tuner is admittedly expensive, but is a precision piece of engineering. 
 
I need to play notes below low-E for a number of different genres, but could never get the hang of a five-string bass. This device suits my purposes perfectly. It comes with clear, concise instructions and is easy to fit. 
 
In use, it is simply a case of flipping the lever to drop the fourth string to D. Its operation is accurate and stable. 
 
I found it so useful that I bought another for my fretless bass. This has the added advantage of facilitating parallel octaves across the second and fourth strings. 
 
Recommended.