Suggestions

this is how i'd do it:

1) adding the quat slot to a dragon that doesn't have it is done through buying a quat scroll from ah / brewing it at baldwin
2) the new quat carries over the same color as the existing tert
3) passing on the quat slot works like passing on genes: dragons without the quat slot are treated as having a "basic" gene, so babies only get the quat gene at a various chance same as any other genes. So like basic quat x glimmer quat = 1% chance of glimmer quat, 99% no quat.
4) quat scatter scroll brewable at baldwin, only applicable to dragons with the quat slot. tricolor scatter scrolls left unchanged.

So, existing dragons are not changed unless the owner "opts in" by buying the quat slot. However, existing dragons can have the quat slot applied by paying.

Support, via buying a 4th slot of sorts. Should be buyable or brewed rather than inherited imo. The tert passed down should be the original one. No pings please - not super interested in discussing further.

I support making this something you have to manually apply to each dragon and is not inheritable. I think making it ancient exclusive would make sense to me, they have far less choices in general. Just as Ancients only have some modern equivalent genes, this extra gene could be really limited to a few options like underbelly, stained, etc.

Of course, a far more simplistic idea would be new terts developed for ancients that combine existing terts (which means they're the same color). So instead of having White underbelly + White points, a new tert would be White Sealpoint that has both underbelly and points applied to the dragon.

I think this could work only if there is a special kind of ancient that supports this mechanic so that it doesn't affect older dragons

So it could be a special variation of ancients where every few years an ancient with the capability for quarternary could be released

It should also be exclusive to ancient because ancients have less customization than moderns due to no apparel

I don't think this could work work on modern breeds (due to needing to be compatible with all moderns at the same time) or already released ancients

I think I may not support OP, but I support Bakugames' version of this.

no support - we already have the ability to create custom skins/accents. if you want to give your dragon peacock and underbelly you can make a peacock accent. dragons on this site already have a problem of looking overcrowded with three genes, accents, and apparel - you can entirely cover a dragon with apparel alone. of course this would also be mechanically complicated, not just visually complicated.

for people that are saying you have to buy a speciality item and it doesn't pass down through breeding: that's basically a skin/accent. a vast majority of tertiaries don't break the lineart. the only problem is if you want two lineart breaking genes on ancients, and there are already clipping issues with them.

as an artist i'm probably biased but dragons are hard enough to draw without any skins and apparel, now yall wanna add a second tertiary gene? i'd cry.

(probably not going to respond to any rebuttals/debates, not trying to start a fight, just sharing my viewpoint.)

i dont support this idea. it would be such a massive undertaking that we probably wouldn’t even see the feature for several years. i mean, devs would basically have to rebuild the breeding system from the ground up for a feature thats not even necessary. sure lots of dragons look bare with lots of genes - thats what skins and accents are there for. or, if you dont fancy those, there are plenty of different tertiaries that cover different amounts of dragon.

and now, some questions. how would the layering work? does the 3rd slot go below or above the 4th? can a 4-slot dragon breed successfully with a 3-slot dragon, assuming we go the “extra customization option” route? what happens then? also, how would 4-slot dragons affect the dragon market? assuming every dragon gets this feature automatically upon release, how does it affect custom progens? wouldn’t that defeat the point of them being almost entirely custom? does the feature apply a random colour, or do you get to pick one? how does this affect completed breeding projects, fandragons and the like?

i just think there would be way too many problems with this to feasibly implement it into the game. it would uproot the entire breeding system that this game has - which i love for its simplicity and ease of use compared to other pet sims!

tl;dr: too many potential issues; no support.