When is a table top too heavy for just four legs?
Sure it's heavy, but it's also stiff, unless it's in terrible condition. You say 4' x 18" - if a typical bowling alley lane (even a "used up" one) it's also 2-1/2" - 4" thick - isn't it? Solid maple, nailed and/or glued up from 3/4" x 4" strips on edge, typically; then sanded down for resurfacing until it's too thin or the business goes out of business.

In short, if the legs will hold the load, 4 is quite adequate - you can probably both stand on the middle of the table, too (again, assuming the legs are up to the task) without appreciable deflection. I would not advise that, but only because the legs and attachment of the legs may not be up to the task - the slab/top should be fine with it.