The RD4000 is a good solid reliable locator and won't let you down for the most part. Personally I use 2 locators, an RD8000 and a Pipehorn 800HL, i've also had experience using RD7000 and Vivax V-Loc Pro 1&2. If I was in your situation I would be buying an RD7000 TL, it is a reliable unit and the peak and null together feature (Vivax also has this) definitely helps to determine how good the quality of your signal is and is very comfortable and ergonomic to use.

If you're tracing a lot of dead services I would suggest a Pipehorn 800HL as well as the higher frequency sometimes finds services that nothing else will.

If you're doing a lot of work such as vertical drilling clearances or deep excavations I would advise you buy a Ground Penetrating Radar, i've used both MALA Easy Locators and IDS Duo and more recently the new IDS Duo Opera. I would advise an IDS Duo as they're easy to use and are great at finding non-conductable services.

I hope this helps!