I often have to re-program my kids after a grandparent trip. First lesson learned - dont' that that again - even if you are at grandma's. :) - I know grandma's don't want any cring and to be able to hold 24 hours a day, but keep the same routine you establish at home.
Second, I agree with the other mom that suggest a growth spurt. 4 months is classic for this. They commonly change routine, needs, and eating becuase of this. It's a whole new stage.
My advise, be patient. The crying it out thing I belive he is too young for. I've done it on my kids but they were much older (over a year) and that going in and out thing only encouraged it more, so if you are going to do it you have to be committed and only when the behavor ir behavoiral.
Right now, do what you can to survive this. Slowly start breaking the new habits, not holding as long, or switching to a swing (this is also the age they really start to love the swing). Just get back into a groove, it might take 2 weeks to do so, but hey we are all slow to change expecially if we like the current routine, so just keep that in mine. Your little guy is a human and love you very much, as you as well love him, so if you hold him a bit extra every now and again cherish it becuase it goes by why to fast.
This too shall pass :)