https://www.opticsplanet.com/sb-tactical-sob-pistol-stabilizing-brace.html

I want to run this brace on my pistol, but I want to fix it as if I had a regular M4A1 stock at full extension, with a pistol buffer.

https://i.imgur.com/6Fq6sUn.jpg

I want to run it like this, but with the brace pulled back to where the length would be identical of an M4A1 stock at full extension when shouldered.

Would it still be a pistol?

It's not a stock. Pistols don't have stocks. Putting it on a pistol will not change it. A pistol can have any length barrel. Putting a stock on a pistol with a barrel less than 16 inches however, is called a short barrelled rifle. You need paperwork and tax stamp to own it. Putting a stock on a pistol with a 16 inch barrel is now a rifle, which can freely return to pistol status. Taking a stock off a firearm made as a rifle first, to make it a pistol is a big no no.. if anyone knows it was a rifle first... ie. bought that way from the factory/ dealer. ;) I think that sums it up... at least how I understand it.. IANAL or expert and things change quickly these days.. or I've been wrong from the get go. ;)
oh.. and there is that foggy area that discusses length of pull... and intent.

Length of pull should not be 13.5" or over. Sounds like your proposed configuration might be too long, OP.