Is he taking the piss ? You might get away with only 5 strings with the missing one at the top or bottom .. in fact Keith Richards frequently leaves his top string ( the bass E string) off. But you would be hard put to play a guitar with one of the inner strings missing.
So ... what is he up to? Well, I remember way way back in the early punk days some reporter visited a Clash rehearsal and Mr Strummer's guitar apparently had only 2 strings.... I think maybe he was trying to make out that he was a poor little starveling who couldn't scrape together the half-a-groat required for a string. I will reveal to you now that you ain't going to have any sort of a rehearsal when the main guitar player only has 2 strings. Turns out he's the son of an international diplomat. He's having a laugh !
So ... maybe GT is trying the same trick ... the "lend me a fiver for a 4th string and I'll play you a nice tune!" But I don't really think that is likely.
I think it's much more likely that he is using some arcane Welsh Mystery Tuning . Yes folks, you can tune the strings of your guitar in all sorts of non-standard ways for many different reasons. Some use odd tunings so that nobody can copy what they play. Who would ever have guessed that the odd-numbered strings were tuned to F and the others were tuned to F# ?
Others do it to make their guitar sound better. My first guitar was bought from a catalogue and it was utter crap... more resembling a cheese-slicer than a musical instrument. I found that if you tuned the strings DADGAD it sounded a lot better because the strings resonated with each other.
Some tune one or both of the E strings down to D ( "drop D" and " double Drop D") ... there's all sorts of possibilities like that which make it easier to play certain chords or chord sequences. Joni Mitchell learned from a banjo book so that might be her reason for using many weird and wonderful tunings.
Anyway, back to the Welsh Mystery Tuning. Apparently the ancient harp players knew certain "mystery tunings" that were believed to charm the "faerie folk" and entice them to do your bidding if you knew their secret. And surely it is no coincidence that as far as anyone knows, nobody has ever managed to work out how Gorky's Zygotic Mynci play the guitar parts of their Welsh-language material *! Because the Welsh Mystery Tuning can only be used to accompany the Welsh language and no other ! That's how the legend goes.
So we have to ask ourselves ... has GT inadvertently given away one of the secrets of the Welsh Mystery Tuning ... the missing 4th string? Or is that what "they" want us to think?
Is it significant that the missing string is a "D" string ? What message could " D" convey ?
Dyfalwch ( more than likely)
Drycin ? ( weather forecast ?)
Dwyiethog ( I expect he is)
Dadwisgwch ( Hardly !)
Dyled ( can't help you I'm afraid)
Dadansoddi ( or maybe gordadansoddi in this case)
Diystyr ( well he would say that , wouldn't he)
Dreigiau ( Where ?)
Diwerth ( probably)
Diwedd ( y peth gorau efallai )
I've also discovered this .....
But what I think is most likely is this ... if you write down what has happened in Welsh ....
Mae'r pedwerydd tant ar goll
"The fourth string is missing" .... I hope.... maybe if you fiddle about with the letters a bit you will find his hidden message! It's worth a try.
Meanwhile, here's that H Hawkline playing Cric yn y Cymylau for us ..
* Except one... Si(mon) Apus .... he seems to know .... have a look at this link ...
http://therockyroadtowelsh.weebly.com/blog/a-suggestion