DC current transmission loss vs. conversion loss
Yes, and in your case Boost/Buck certainly will be less efficient unless you are using extremely low voltage/high current for your application. 

DC-DC convertors tend to be no more than about 90-95% efficient, so placing one at each end of your 4 m cable will result in approximately 10% loss.

In addition, the Ethernet cable has a resistance, so there will be an additional loss in the cable. The fact that you boosted and bucked the voltage does not eliminate these losses, however conversion to a higher voltage will reduce the I^2R losses.  

7 W @ 5 V is about 700 mA.
Typical Ethernet cable is about 200m Ohm/m (loop), so voltage drop will be about 0.15 V/m.
If you can tolerate 150 mV/m  drop in your 5 V (about 3%), then you don't convert. 
If you need regulated 5 V, then start with say 6 V and put a low dropout regulator at the application end.  

EDIT:

If you want to consume 5 V @ 4 A at the remote end, then it does make sense to boost/buck.
Resistive losses at 5 V would be close to 3.2 V .....so not possible.
If you boost to 30 V (about 700 mA current for 20 W) then the voltage loss will be about 0.56 V.
You then Buck down to 5 V at the remote end. 
Likely you will have about (let's be conservative ) 90% efficiency in each convertor, so about 80% efficiency overall. 
You need to consume 20 W but need to supply 25 W of input power to allow for losses.