driver needed from Milan airport to northern Tuscany

Google for "transfers" from Milan to Tuscany. You will find several options. I am doing the same thing from Rome to Umbria.

The cost of a private transfer on a Minivan from Milan (MXP) to Florence is approximately €500 (US$700). For 6 people that would not be a bad deal (about €85 per person), as 6 high speed train tickets would cost about the same. You can google NCC Malpensa Firenze and you'll find several companies. NCC stands for Noleggio Con Conducente (Car hire with driver).
However you have not posted where your rental home in Northern Tuscany is. If it's a farmhouse or a house located in or near a small village, you will be needing a rental car (or two rental cars) to go around because public transportation (including taxi) may not be a viable option where you are.
If that is the case, it might be beneficial to rent a van or even two small cars (two economy cars are sometimes cheaper to rent than one van) at the Milan airport and drive to Tuscany yourselves. The road from Milan airport to Tuscany is 100% freeway (just over 200 miles in total), except for the portion from the freeway exit to your house location, which probably is just a few miles. In any case the drive will be almost totally on a freeway similar to the ones you are used to driving in America. The freeway from Milan to Florence is 6 to 8 lanes wide and they are even much better maintained than US Interstates.

The cost of renting an economy car (similar to a Ford Fiesta) is about €40/day (US$55) including no deductible insurance. So in other words with the cost of hiring a private driver for a 3.5 hour drive from Milan to Tuscany you can rent 2 cars for a week. The cost of fuel in Italy is high (about $9/gallon) however distances in Italy are small and gasoline will not affect your budget much.

A diesel car (most cars in Italy are diesel) will need just about 20 liters of diesel fuel from Milan MXP to Tuscany. At €1.70/liter (the current price of diesel fuel) you will need less than €35 (US$50) in fuel per car. The freeway tolls from Milan to Tuscany will add another €25 max to your total cost. So even if you rent two cars because renting a van might be more expensive than 2 cars, the cost of driving from MXP to Tuscany will be approx. €200 (incl rental+fuel+tolls) instead of €500. In addition you won't need to hire a driver for 4 days of touring, because you will have your own wheels. Each day of touring with a van and driver will cost you hundreds of Euros. For example an 8 hour tour of Tuscany from Florence including Siena and San Gimignano areas with a van with up to 7 pax. ranges between €300 and €400 (US$400-550). A 4 hour tour of Tuscany will be around €200 and up depending on distance. You can rent a car and buy enough fuel to drive all day for €50 per car.

Oh my, oh my you have been so helpful! I have tried so many ways to get this info to no avail! Thank you!!!

If you decide to go with rental car(s), note that each driver will require the compulsory International Driver's Permit, which is used in conjunction with your home D.L. You'll also have to be vigilant to avoid the dreaded Zona Traffico Limitato (limited traffic) areas which exist in many towns in Italy (expensive fines!).

Be sure to pack along at least a good Map if you'll be driving, or preferably a GPS unit.

Cheers!