Carefully drill the head off and then use pliers to get out the rest of the bolt - not fingers as the tip of the bolt is going to be sharp.
Or
Hammer a torx bit into the end making sure it's a tight fix.
Tighten the other 3 bolts up a bit more relieving the pressure on that one bolt allowing you to undo it a bit.
@O'Shane started
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Carefully drill the head off and then use pliers to get out the rest of the bolt - not fingers as the tip of the bolt is going to be sharp.
Or
Hammer a torx bit into the end making sure it's a tight fix.
Or
Tighten the other 3 bolts up a bit more relieving the pressure on that one bolt allowing you to undo it a bit.