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I don't know about the S4 but on my S5 I've never taken the front bumper off to remove or install the FTP lenses. It's been a while since I did it but I don't remember taking the trim off the headlight. I think there is a plastic piece beside the headlight that after removed and with the headlight up you can stick your arm in there and get to the nuts that hold the FTP lense on. It's done mostly by feel.
I don't know about the S4 but on my S5 I've never taken the front bumper off to remove or install the FTP lenses. It's been a while since I did it but I don't remember taking the trim off the headlight. I think there is a plastic piece beside the headlight that after removed and with the headlight up you can stick your arm in there and get to the nuts that hold the FTP lense on. It's done mostly by feel.