This is where i feel there is a lot of uncertainty. Because base weapons seem like they are modded into other weapons the variety is much higher.
For instance it would appear that the Laser Pistol is a base weapon (or just, Laser as a base weapon). And through modding you can get different varients of Laser Pistols, you can mod it to be a Laser Rifle, or a Laser Sniper, or a Laser Scattergun. And all of those would fall under just one base weapon.
In Fallout 3 that would've counted as 4 seperate weapons atleast. The Laser Sniper didn't exist for example, but the Laser Pistol and Laser Rifle were two seperate weapons. So what counts as one in Fallout 4 would've counted as four in Fallout 3. Might even be more potential distinct varients, Laser Assault Rifle? Think i saw that one.
Some varients would of course be closer to eachother than others, i dont think there is a large difference between a Laser Rifle with regular stock, and a Laser Rifle with a combat stock. It does matter for the stats of the weapon of course, but gameplay wise they will probably be fairly similar.
So the total pool of weapons would appear to be significantly larger. As for fun weapons, we haven't seen all 50 base weapons yet i think. But the Junkjet can be modded to ignite or electrify the junk it shoots at enemy, so you can blow someones head off with a burning teddy bear. No idea how and if this will work with Unique weapons.
It is just a bit tricky to discuss this because i am honestly not sure how exactly they count base weapons. I assumed Laser was a base weapon and Pistol and Rifle were part of that, but i couldnt tell you for certain they truely are not counted as seperate base weapons.