thanks for the comments. I just returned the SD card (Staples) and decided to get a direct replacement. I had used ~ 16 Gb of the 32 capacity but the content was mainly stuff copied from a laptop so luckily backup wasn't a problem and my default storage (for photos etc. taken with the S4) was internal storage so didn't loose anything. After that sober wake-up call (card failed after ~ 4 weeks), I'll start by making an exact folder replica on my laptop called "SDCard" and cpy to the SD. If it fails again, it will be easy to replace. I have 3 backups of really important stuff: laptop, Seagate 1 Tb Wireless PLus and a few 250 Gb WD drives.
I wonder about that S4 SD card socket ... it just seems slightly flaky in terms of a really good electrical contact but maybe I'm just getting mopre paranoid.