Stabilize metal legs attached to wood table top

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.

It would be helpful if you could add a picture of your desk to your question. The commenters are correct. A work table with 2x4 legs is probably not fancy, and doesn't need a fancy job to raise it. If it were my project, I would either replace the legs with longer ones, or add a 5-1/2" stub under the leg while scabbing a full length 2x4 leg to the inside of the original leg. Use 2-1/2 inch screws and horizontally screw the new leg into both the old leg and the stub. You can optionally toe screw from the original leg into the stub - just like a pocket hole, but less of a process.