Sheldon Brown has a gear calculator available so you can input all your info there as-is, note the gear inches for gear 4, and then change the ring size until your modified 8th gear is near that number. In addition to the info you've already provided, you'll need the number of teeth on your hub's cog to place in the spot of the first cassette cog.
FWIW, I recently had a dawdling bike with an internal 3sp and the smallest ring my cranks would accept and I sometimes wished for a lower gear. Also if your setup looks like this then you may not be able to downsize very far without swapping cranks.
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How small can I get and what tools do I need to replace cranks? Sheldon Brown's BCD crib sheet can let you double-check your measurements so far and it also lists the smallest ring any particular configuration will accept. The interface between your bottom bracket and cranks is almost certainly square taper, so you'll need a new crankset with that same interface (which is a combination of quite old and quite common so that's good news for you). The only tool that you should need is a crank puller to get the existing cranks off.
Can I use mountain cranks? Assuming it's got a square taper bottom bracket interface, you can go with any cranks you want. Especially with a multi-gear crankset you should mind your relative chain line. My hunch is that the smallest ring on a triple in your scenario could be sufficiently inboard of the cog for the chain to work its way off the outside of the ring, seeking the plane of your cog. In that case you could swap your bottom bracket for one with with a longer spindle. More tools and expense, but possible.