First of all, welcome to MDC!
It sounds like trying to get your DD to sleep on her own has been a big stressful hassle for everybody concerned. So it would take a lot of pressure off you and yoru dd if you just resigned yourself to cosleeping for a while! I think you will discover that when you accept this is what you need to do, just accepting it will be a big relief. Because to me it sounds like trying to do something other than what works is just creating stress for all of you. Why not just go with what works?
So, if you are going to take that step and decide to cosleep there are many things to consider. First, why is your DH in the guest room? Is your bed too small? You might want to invest in a bigger bed, or add a mattress beside your bed, or put your bed on the floor. This way you don't have to be away from your DH at night, which I agree must kinda suck! Second, no your DD will NOT necessarily be in your bed for the next three years. It's up to you to decide when it's not working out, but it would be alot easier on you and especially your DD if you waited until she was alot older, and able to understand what you are doing and why. There are many gentle ways to wean, like Pantley, but I believe even she says to wait, doesn't she? Four months is too young, IMO, to wean or try to sleep-train.
Also, you want to know what to do when she is ready for bed and you aren't. Well, have you considered having her sleep in your lap until you are ready to go to bed? I do this sometimes if DD goes to sleep before I'm ready, and it's really relaxing to just sit and read a book, watch TV, or surf on my laptop with no interruptions. Or sometimes DH and I will watch a movie together. As for naps, you might have to do the same: have her nap in your lap or in your arms and resign yourself to the fact that, for the next little while, things you "plan" to do when DD is napping will have to go on the backburner. I accepted this and did not plan to do things when DD napped, and now that she is older (9 months) I am able to easily lay her down for naps and get things done.
I'm saving this next bit for last, but really it's the most important thing I have to say: Four months is pretty darn young. I don't know what your expectations were regarding infant sleep, but I think you need to recognize the reality of infant sleep and that you might be expecting too much. I can also tell you that your dd will almost certainly change her sleeping habits and so things will not always be this way. "This too shall pass", as they say. In the meantime, be there for your DD and give her what she needs, to the best of your ability. One day you'll look back on these days and wonder where they went. I suppose the best thing you can do to convince yourself to "let go" is to recognize how much stress and trouble you have gone through in your efforts to get DD to sleep the way you want her to, and how much easier it would have been to just accept it and go with it.
Well, I hope this helps and doesn't sound too preachy!