4 oz. jars will work fine, although you're right, you'll only produce roughly half as many shrooms per cake. Fruiting is relative to the amount of substrate used, so 1/2 substrate=1/2 the yield. You will produce your shrooms quicker with 4 oz. jars, but you have to do twice as many to get the same results as 8 oz.As far as inoculating a jar from a spore print goes, you're on your own. I always use a syringe. Maybe someone else can help you there. I would imagine you'd need to start a mycelium culture in a petri dish, then somehow get the mycelium into the jar? Try the MMGG for some ideas there.
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A little nonsense now and then is relished by the wisest men.
---Willy Wonka