To the best of my knowledge, the only Sierra related fangames that received C&D from Vivendi or Activision were the fan made Space Quest VII project and The Silver Lining (back when it was called King's Quest IX), back in 2005. I do not know whether the graphical aspect of the projects (both were going for 3D graphics) played a role in the C&D letter but most certainly the title itself and the numeral that accompanied it was a pretty big deal. Both projects wanted to attract the attention of as much fans as possible (for extra visibility but also to attract as many potential artists, coders and developers as possible) and went for those specific titles and that rather important numeral to accompany them and that attracted the attention of a lot of fans but that also attracted the attention of Vivendi at the same time. Both projects were made an offer:
They had to change the title of their project,
They had to agree that the project would be 100% owned by Vivendi/Activision on release,
The King's Quest IX project agreed and changed the title to The Silver Lining. The Space Quest VII project however refused the offer and cancelled their project.
And while AGDI's four remakes were released with the permission of Vivendi/Activision, you'll most likely remember that Vivendi didn't seem to care when Infamous Adventures released their King's Quest III remake without their permission, nor did Activision break a sweat back in 2012 when fans released three Space Quest projects during the same month: A remake to Space Quest II and two sequels to Space Quest VI that didn't feature "VII" in their titles.
So, the chances that your King's Quest IV remake receive a C&D letter, if you're not claiming to be an official sequel, if art wise it looks like a fan project in Activision's eyes and not a game that would make their official sequels in the series look bad, if you don't try to make money out of it... those chances are pretty slim, and worse case scenario, you can most likely try to negotiate with Activision to get their permission, the same way they allowed AGDI's four remakes or The Silver Lining to release their project. Plus, you've been keeping such a low profile that myself, being a fan of those projects, I never know whether you're still working on the game or whether you moved on to something else, and big companies tend to not waste lawyer money for low profile projects that, in their eyes, may simply disappear on their own.
They had to change the title of their project,
They had to agree that the project would be 100% owned by Vivendi/Activision on release,
The King's Quest IX project agreed and changed the title to The Silver Lining. The Space Quest VII project however refused the offer and cancelled their project.
And while AGDI's four remakes were released with the permission of Vivendi/Activision, you'll most likely remember that Vivendi didn't seem to care when Infamous Adventures released their King's Quest III remake without their permission, nor did Activision break a sweat back in 2012 when fans released three Space Quest projects during the same month: A remake to Space Quest II and two sequels to Space Quest VI that didn't feature "VII" in their titles.
So, the chances that your King's Quest IV remake receive a C&D letter, if you're not claiming to be an official sequel, if art wise it looks like a fan project in Activision's eyes and not a game that would make their official sequels in the series look bad, if you don't try to make money out of it... those chances are pretty slim, and worse case scenario, you can most likely try to negotiate with Activision to get their permission, the same way they allowed AGDI's four remakes or The Silver Lining to release their project. Plus, you've been keeping such a low profile that myself, being a fan of those projects, I never know whether you're still working on the game or whether you moved on to something else, and big companies tend to not waste lawyer money for low profile projects that, in their eyes, may simply disappear on their own.