The chance of getting a precursor from four rares is less than 5%. My experience:
- Rares (Lvl. 80) placed in the Mystic Forge: 1,763.
- Exotics obtained: 100 (5.67%).
- Precursors: 0
Perhaps the chance increases with exotics. I haven’t tried.
Is the 1,763 the number of tries, or the number of total weapons used? If its the number of tries, then you should divide by 4 first. I’m thinking that this is the number of tries since it is not divisible by 4, but earlier in this post someone said it was roughly a 25% yield for an exotic from rares. 1,763/4 ~ 441 and 100/441 ~22.7% which is fairly close to 25%, so you might be able to see the confusion.