How can I get a longer Samsung USB data cable for Galaxy S4?
Based on my understanding of the multiple posts on Internet dealing with charging of S4 :


The Charging adapter has it's D+ and D- pins shorted with resistors across them. This is done  to ensure that higher current does not flow , reducing chances of damages to the device (see Fizz's answer on electrical.SE and this XDA post
This is NOT non standard but less frequently implemented (Apple legacy continued with few Android OEM's )
This affects (or should ) charging only 
USB cables should not have anything to do with data transfer when connected to a PC . The only reasons I can think why this is happening are :


Non OEM cables purchased may be poor in shielding - XDA post linked above mentions this , though on context of charging ( I am not entirely sure how much it could affect )
They happen to be all Power only no data aka USB condom cables



Both possibilities are debatable 

To eliminate the possibility of shielding affecting , suggest you to consider the cables mentioned in my answer here . They are very sturdy and 2M long and give excellent charging performance too ( I am not canvassing for them - totally satisfied customer )


USB cables of Samsung are also modified in some way , and this prevents other USB cables from transferring data - this is more likely but haven't come across any solid evidence to support and will update if I do find something