Depending on how many neighbors you have, you may be required to add a few to your game before being eligible to purchase the Polar Parcel for coins (it costs 50 Farm Cash otherwise and isn't on sale). Once you do reach the required amount of neighbors (if you have to add any at all), you can expand your farm for 1.25 million coins. Considering that that's cheaper than some animals released in the store for coins, it's really a bargain, especially if you're a heavy user of your new fourth farm.
There isn't a time limit for this coin release, so it's a permanent addition to the game's store. If you don't have enough coins or neighbors on hand right now, at least you won't have to rush to add them to access this expansion. Not worried about expanding right now? Just play as you would normally and they'll come to you in time.
Will you immediately go to work on expanding your Winter Wonderland farm to the 18x18 Polar Parcel, or have you decided to only use this fourth farm when the Christmas season arrives again next year? Sound off in the comments.