Yes it definetly is. The two sets of pre-outs will be marked Front and Rear. The front set will go straight to the 4 channel powering the four speakers. Hook the RCAs up to the inputs marked Front. Then run the RCAs out of the REAR marked outputs on the Headunit and then split the signal using Y-Splitters to feed a signal to the REAR channels of the four channel and to the main input on the sub amp.
Make sure that they are both outputs and not an input and an output.
Also have a look to see if your sub amp also has a pre out. If it does then use the ysplitter. Simply run the rears straight into the sub amp and then from the RCA output on the amp into the rear channels on the four channel.