I've finally used my Moonman (Majohn) C4 enough to rate it. Iike nearly all MM pens I'll give it a 10/10 if I'm feeling generous.
Nib: Like the Q1, comes in extra fine and writes well, cannot compare to fine/med nib since I always go for the finest available, but I assume the fine nib will be the same as the C1/C2 (I seem to recal at least the finer nibs on C3 were bad, but those were gold coloured and very different) Either way I'm happy to assume you will get a smooth writing experience, with decent flow. For me, the EF nib being offered is a huge bonus. No issues with the feed so far, though I'd say it's a little dry for fast writing I've had no skipping, only paler/thinner lines if I write fast. As it's on the dry side, there's been no leaking so far.
In terms of both nib and styling it resembles a stretched out Q1. If you liked the design of the Q1 but found the form unappealing/uncomfortable to hold, this may be the ideal pen for you.
Filling: Like the majority of Moonman pens, it's dropper fill, according to the instructions that come with it. HOWEVER, It is, for some reason unfathomable to me, designed to look like a vaccuum fill pen. This design goes far enough to have a functional movement: an uncrewable end and vaccum pole built into the body that really pulls out... but without the diaphragm that would allow it to vaccuum fill properly. Out of interest I tried. This was what's called "a stupid idea" and probably wasted a few droplets of my precious Dominant Industries ink.
Frankly I find this descision to be both cynical (aping the look of a popular fill style that tends to command more money) and ugly (it's ugly). Once the pen is filled with a dark coloured ink this faux vaccuum pole doesn't really show, so the only sign of the design is the extra threading at the end, which I don't find to be ugly. Be cautioned that with lighter coloured inks or when the pen runs low on ink, the piston might be visible and unappealing. This detail with the usual visual style of a transparent acrylic Moonman, which tend to be droper fill precisely to create that minimalist look. Still, the solid acrylic body is very beautiful, as they always are.
Since Moonman have never had a vac fil pen in their roster (as far as I'm aware) I suspect this was initially a sincere attempt to create a vac-fill mechanism which simply failed at some stage of R&D and rather than waste their product development money they released it as an oddly designed dropperfill pen. I prefer dropper-fill anyway. I find vacs fiddly, difficult to clean, impossible to top up and unpredictable. The lack of vac fill is a bonus to me, as I tend to use a dropper to fill my Wing Sun 3013s and lament needing to use the vac on my WS 601A, which is the only vac pen in my live rotation.
Lid is screw cap (this probably gives Moonman their resistance to drying up in storage, which is why they are ALWAYS in my ready-to-write deck, so any inconvenience is well compensated!) it posts somewhat securely. (if you're not a fountain pen nerd, posting is our special word for putting the cap on the back of the pen) being quite solid in its build and acrylic, the cap is weighty and when posted may not suit a writer who dislikes top-heavy pens. Unposted the pen feels a touch light, but well balanced. I'm not very sensitive to these things. Its construction is reasuringly thick in the acrylic and with the cap on it feels heavy and prestigious. Though some of the gold trim is a touch gaudy it is finished well without any flash/burrs/other signs of poor moulding, and it doesn't have the vile tackiness of the S5's plastic gem. As it's the same as the Q1 in many respects I expect this plasticy gold trim and clip will wear the same, mine showing thin spots in places and minor scratching which actually helps take the excessive shine off.
Due to impatience I ordered my C4 from already imported European stocks, rather than save money with an overseas order and potentially get the Asian market version what still says "Moonman". For this reason the barrel trim on my pen says Majohn, the European trade name since Kaweco patent trolled them. The nibs tend to not follow this rule, wether due to the specificity of the patent troll, or simply hoping no-one notices (don't tell Kaweco).
I hope you found this review interesting. I'll happily review more pens if anyone actually reads all of this and wants another.