Since LR 6.0 is the first version to support the TG4, version 5 will never support it.
So, yes, you will have to upgrade to LR 6/CC or use the stand alone DNG converter.
Now that Lightroom CC/6 has been released, there will be no further development on Lightroom 5. That means that Lightroom 5.7.1 will be the final release. However, if you don't want to upgrade, you can always download the DNG converter 9.0 and create DNG copies that you can use in your version of Lightroom.