Prolly doesn't mean much coming from me because I built the cue in question. BUT.....
I would consider 4.2 a heavy shaft. I have had shafts that weigh around 4.5oz but those are just cumbersome. I personally prefer to use 3.7-4.2oz shafts in my cues. I think taper shape, joint type & diameters play a big roll in shaft weight so there's not necessarily an easy way to say what's heavy & what's not. For my taper & joint & 13mm shafts, 4.2oz is edging close to as heavy as I feel comfortable using. Other cues would vary & would have their own specific window of prime weight range. But overall in the big scheme of things, anything 4.0oz & over is generally considered heavy for any cue.
As for good or bad, it's all about the cue & how well the shaft works with it. If it weighs 4.2oz and balances out nicely at a playable weight, then it's a good thing. If it's 4.2oz and makes the cue weigh 22oz with a 22" balance point then maybe it's a bad thing & a lighter shaft like 3.2oz would be best. It all depends on the cuemaker & build methods he uses. For me personally, 19-19.5oz, 19-19.5" balance, 13mm, 58" is perfect and for my build formula a 3.7-4.2oz weight range is where I like my shafts.
Hope that helps. And i'm sure you'll get folks telling you how heavier shafts play better but IMO, that's BS. A 3.2oz shaft can play just as solid & lively as a 4.2oz shaft. Took me years to learn that but knowing now what I never knew back when, nobody can convince me otherwise. I choose my shafts by tone more so than anything else because that is the best way for me to determine how playable a shaft will be, regardless of weight. Light shafts range in tone just as much as heavy shafts. If I used brass inserts in my shafts, low 3's is what i'd be targeting for my cues because with an insert they'd give me my 3.7-4.2oz that works correctly with my build techniques. Again, hope this all makes sense.