The max current from your battery is the capacity in Ah multiplied by the C rate (in this case you have a burst and continuous C rate, so use the lower of the two, i.e., the continuous rating). That's 4.0 Ah x 60 C = 240 A. The fuse must be less that that.

However, with two drill motors and a linac you'll never get close to 240 A. So to protect your motors and ESCs you are probably better off with something around the 100 A mark, or even less. We have an 80 A fuse for a similar setup to yours.