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Probably the FD 300/4L will be your best bet here.
The non-L might be just as sharp, but the FD lenses really need the fluorite or UD elements on m4/3. I have an FD 200/2.8, which is very sharp wide open, but on high contrast subjects, has really severe purple fringing. My FD 50-300 f/4.5L, however, is sharp and shows very little CA or fringing at all. In a perfect storm of conditions, it can exhibit a mild blue fringe, but it's very rare, and very easily taken care of.
The 50-300 f/4.5L is a fantastic lens, but it doesn't fit your qualification of 'compact', as it's quite a beast...weighs 4 lbs and is pretty huge: