4 months is a LONG time to put a dog in a kennel. A kennel usually charges $15 to $30 per day, optional walks, baths, medication at extra fee. Some kennels provide long-term care (more than a month) at a discounted price, i.e. $11.50/day, but for 4 months, you are still looking at a bill that exceeds $1300 dollars. So basically you are looking for a foster home for 4 months, that's a lot of responsibility.
I did that last summer for someone, also for 4 months. They had an 8 month old golden retriever Oscar, very cute, and we had a lot of fun with Oscar, also a lot of work, even though we enjoyed it very much. The thing with putting your dog to a sitter is that you won't know if the family would treat your dog the same way as yourself. In retrospect, because we were first time foster parents, there are a few things we could have done differently on Oscar. The good thing was: we were RESPONSIBLE and loving people (self gloat here, pls bear with me
).
So as a dog owner, you should really spend some time with potential foster parents, visit their homes, talk with them and make sure your dog would be taken care of in a responsible and loving family. As for costs, ask the foster parents to keep all receipts and you'll reimburse when you return, or leave some money up front and reimburse the rest when you return. We kept all major receipts for Oscar, but for many little things like treats, toys and bathing products, we just bought on our own. Money should be the least of your concern, since if a family is willing to take your dog, they probably won't really care about money. Imagine the amount of work and time they'll need to spend on your dog, and the responsibility, so it's really a BIG FAVOR.
One last thing, before you leave the country, make sure to have your address on your dog's registration transferred to the foster family. We were told by the city of Ottawa animal by-law officer that Ottawa animal by-law requires this. Also if the dog is lost (God forbid), the founder can contact the city and they can get in contact with the foster family instead of with you, who are thousands of miles away.
Good luck to you!