Helo
With breton language, I’m used to pronounce added letters coming for example in front of a vowel, in order to make an artificial but “pretty” result (and also “easier to say”. They say…
So, as welsh, we for example have “ha” and “hag” for “a” and “ac”.
Ar c’has hag ar c’hi (the cat and the dog)
But
Kas ha ki (Cat and dog)
What I don’t understand refers to challenge 4 (north) with Dw i and Dw i’n. It seems to work in order contrary of “a” and “ac” (or other words of this kind) :
We (or I, at least !) woulld expect to have the N before a vowel, for example "Dw i’n angen, or even more “Dw i’n iisio” (to avoid the “hiatus” between the two i) and not needing this N in front of a consonant. I don’t find easier to say “Dw i’n mwynhau” than Dw i mwynhau (I would even find more “complicate” to say Dw i’n mwynjau)
But it is the contrary
But mayve this N has another use than the tradittonal - and often true but not always ! - explaination "easier to say this way, more natural etc ? 
So : why those Dw i’n in front of a consonant, and Dw i in front of a vowel (if I correctly understood challenge 4, otherwise I would be ashamed asking a useless and ridiculous question !) ?
Diolch yn fawr