You can do nothing with your present transmission to change it from a 3 speed to a 4 speed.
YOu will need another different transmission. The 700R4 just about bolts right in but you need to fabricate a rear mount and shorten the drive shaft plus make a stand alone torque convertor lockup.
The other alternative is the 200 R4. It can be make just as strong and is even easier to install. It is becomming more popular for swapping.
Both bolt right up to the motor without any modification and both clear the tunnel.
The 200 also has a better spread of gears then the 700.