In a perfect scenario, an UNFINISHED attic as well as a finished one it would be 'very practical' to have soffit vents and a ridge vent installed for ventilation AND to keep the attic roof cool as well as the attic 'space' cool. I HAD (note the past tense) your particular setup until I installed vinyl siding. The only difference was I ripped out the wood that's called the Soffit. Your Contractor is suggesting that holes be drilled every 4 feet or so BUT the RIGHT way is to have ventilation holes between EACH AND EVERY set of roof rafters. In a sense, your Contractor is suggesting you only 'vent' essentially EVERY OTHER rafter. I don't agree. Suggest he vent EVERY set of rafters or (as I did) rip out the soffit (wood) and install Vinyl Soffit. Again, he's recommending a short cut. IN short, I'd vent anyways.
Instead of trying to say too much, poke around these sites
If you have more ?'s, post back. Again, in short, DO vent the attic AND vent between EACH and EVERY rafter (either by drilling or by removing the existing wood) and installing Vinyl Soffits. My best to ya and hope this helps.