[storeman]

RCA inputs are usually line level inputs rather than speaker outputs. There are no line level outputs on the RD4 so you risk at best distortion to worst case blowing the amp if you use the speakers as the input.

I'm not certain of this but the speaker - lines may not be ground as the outputs may be a push-pull arrangement to get a higher power rating so if the - input of the amp is a ground you also run the risk of blowing the head unit power amp chip. You may need to add some kind of speaker to line level converter.

 

[storeman]

From the advert you linked to.........

Installation Guide
1 x Black = Earth
2 x Green = Left Positive and Negative Speaker Connection
2 x Grey = Right Positive and Negative Speaker Connection


You need to know what the amp will accept as inputs before you go spending money on additional equipment. I bought active speakers a long time ago with each speaker having it's own 50 watt amp. These accepted speaker level inputs as each speaker amp had it's own attenuator built in. If the amp you have or are intending to buy is just for the sub then it's likely to accept speaker level but if it's a four channel plus sub then most of the ones I've seen from the dark days when I wanted more from the factory standard than it could provide were line level - but that was quite some time ago

You need to get the specifications of the equipment you intend to fit before anyone can give you a definitive answer. Without this information it's all just guesswork.