Re: Adopting a 4 year old lab?
The dog will be depressed for the first few weeks (my experience was about 3 weeks). Literally whining, crying, may even try to escape for old owner. Can't stress enough the showering with affection as early and as much as possible. Toys and play-time are the antidotes. Let the dog explore as much of the neighborhood, dog park etc. in a controlled setting with walks etc. And, as stated above, be firm with rules (if there are multiple members in household make sure every person is consistent with rules - such as no begging, where the animal can/can't lay, amount of barking permitted). Since you know the owner, you have the advantage of learning the dogs' pre-existing commands. No reason to change them. See also if the old owner will allow you some of the dogs "comfort items" - old toys, blanket from crate, etc.
The benefit is..usually they are housebroken. Here is a suggestion: put some jingle bells around a door knob (even more than one door works, but do make sure it include bells to the backdoor or yard). Jingle the bells before you take him out to relieve himself. If you are consistent, he will learn to tell you when he "needs" to go out by jingling the bells.