You say you are getting spark at ALL plugs. That would cause me to direct my thinkingaway from things like distributor cap, rotor, and plug wires. It could be bad plugs, but that is easy to confirm by swapping them with functioning cylinders.

If there is a problem with intake-associated items like the AMM, vacuum hoses, brake booster, etc. it seems to me it would affect all cylinders.

Before you throw a bunch of money at new parts, you might want to check the compression. It almost sounds like you've got a head gasket leak between #3 and #4.

Another possibility is a leaking intake manifold gasket. You could check that the intake manifold nuts are all snug. You can also squirt a little oil around the gasket at the #3 an #4 intake runners to see if it temporarily smooths the idle.