Just wait until you do find one you can a/b it with. Don't make any hasty decisions....I jhave almost always regretted ones I have made regarding gear.
I've been thinking of picking up a Mark IV as well. I really want a Mark III blue or green stripe but the prices have been getting high and it's getting hard to find one in good condition so fuck it...for a few hundred more I might just get a Mark IV. I'm going to see Lamb of God in a few weeks and that will be the deciding factor since I never see any in shops around here.
I'd wait until you get a chance to a/b the two. This is just a normal 'gas' stage you are going through...it'll pass. There will be plenty of Mark IV's in the future as well as buyers for your Herbert.