Probably not much you can do about it - my hands would hurt too if I played spider patterns for 4 hours. The only chance to get rid of it is to alter your hand position and see if you can fins a place where it goes away. If it starts to hurt quite soon, it might be worth looking at how tight you are holding the neck - Are your finger tips white? I've seen it happen.... Thinking about it that's the most likely explanation for thumb pain.
Try varying your practice a little so you're not stuck in the same position for too long would be my advice. Also, if you were practicing 10 minutes a day and have gone straight to 4 hours+, you'd be better off increasing gradually...
Should also mention that 4 hours of spiders a day might not be as useful overall as an hour on technique (alt, sweep, legato, tapping etc), an hour of theory stuff, and hour of chords / rhythm and an hour of jamming to CDs having fun. Remember it's supposed to be fun - I know I manage to forget sometimes.... :rolleyes:
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