To obtain the M4A4 Asiimov you can attempt to upgrade a Golden Minigun into a M4A4 Asiimov at ::upgrade, which has a difficulty rating of medium. You can also purchase it from DreamScape's Webstore for 250$. Players can also buy it from the Slayer Level 2 Prestige store, for 5000 Slayer Points.
Currently, it is not dropped by any bosses.
It can be obtained from the following boxes:
At the moment, the price of M4A4 Asiimov is roughly: 10M-20M DScoins