Zero turn mowers are my specialty. I've been a member of Lawnsite since May 30, 2001 and have owned a couple dozen ztrs. I've cut for money and I've bought and sold mowers because it's an easy way to make money when you have the location and know how. Dizzy Dean said it's not bragging if you can do it and I can do it.
After that big intro here are my words of wisdom - most all ztrs are well made so buy the dealer. I see you are in Montana so you probably don't have multiple power equipment dealers nearby - find the best one that is most convenient and give them first shot.
For 4 acres do not consider a 42" deck. You would hate it and trade soon. Buy once, cry once. 61" deck is what you need and 25hp is adequate - Kawasaki 730cc is good, 801cc is better. Unless you are bushhogging you don't need the gas hog 1000cc. FR/FS series engines are good, FX series are best. I'm talking Kawasaki - Kohlers, Briggs Vanguard and the proprietary engines some mowers use are good but Kaws lead the industry and parts are easily available.
I've owned Dixie Choppers, Big Dogs, Hustlers, Bob-Cat bought new and many other brands that I've traded on - I've never had a bad mower but I don't buy junk. In a population center like Louisville used mowers are plentiful and craigslist is a good place to look. Estate auctions too but I do better one on one vs auction fever.
If I'm mowing 4 acres I'm buying a Hustler Z series 61" or Big Dog Diablo - same mower just red. I'd look first for a well kept used machine w/under 200 hours but a well kept machine with under 1000 hours still has a lot of life left, just buy it right.
If the deal is on an Exmark, Kubota, Scag, Toro, Grasshopper, Ferris, Deere, Bad Boy et al, take it - major brand commercial mowers are all good as long as they have not been abused but I like Hustler best - made in Kansas by Excel industries, one of the first to build ztrs and they perfected the design long ago and they don't change specs just for the sake of change - keeps parts plentiful.
Do not overlook a Hustler Raptor if your 4 acres isn't super bumpy - homeowner grade but with a welded deck, Kaw engine, serviceable transmission and comes in deck sizes from 42"to 61". I'd still hold out for a super Z or Diablo but a Raptor will make your yard look good and won't break the bank. I use a Big Dog 48" fastrac on my 3/4 acre. 30 minutes if I piddle around.
This is my Big Dog with power bar steering, same as a Hustler Trimstar. twist either hand grip forward for forward and backward for reverse. Easy to steer one handed and will turn in it's length. Best mower Hustler ever made that was not a commercial success. I bought this new in 2012 for less than the price of the Kaw FR651/21hp and the Hydro Gear 3100 transmission - helps that I've bought about a dozen mowers from SIE power equip - I get cash deals on used equip trade ins and best price on new mowers. At age 71 I'm expecting this Big Dog to last me as long as I care to use it considering it only gets used about 25 hours a year.
