You do not have to spend time learning the 4th and 3rd strings individually (although in time this will happen without your realizing it) as you can use the 6th and 5th strings as the point of reference for them. All that needs to be done is to learn the octave shape and your 6th and 5th string notes. You now have the knowledge to map out 85% of the fretboard!1
Alternatively, you could use a different pattern for playing octaves from the 5th string to the 2nd string (fig 1.3), where the 1st finger is playing the octave to the the 3rd finger. This method relies on the fact that you know the 5th string notes well as discussed in Step One.
1. From The Essential Guitar Guide.