My heart goes out to you. 17 months of no sleep - and a 4 week old thrown in for good measure ?!? You must have the patient of a saint.
I do have a little bit of advice for you - although, my experience is based on 1 child -so, others may be better placed to advise you than I am.
I think the first thing you need to do, is to get your 17 month old out of your room. My ds is 14 months old - and whenever we go away, he sleeps in our room in the travel cot - and he always has the worst nights sleep. I think the reason is, when he wakes up in the night (and we all wake up through the night), he can see us, which means, he wants attention or wants to play. Whereas, when he wakes up in his own room, there's nothing going on, and he sends himself back to sleep. Might be worth a try ?
Also, I would do my damdest to make sure you don't have the same problems with the 4 week old. The reason being - if you have the same issues again - you're not going to have a full nights sleep until 2009 !!!
A friend of mine conquered her issues with her 22 month old when her second lo was about 10 weeks old - by putting them both in the same bedroom, and explaining to the older one, that she needed his help in looking after the baby at night - i.e. if the baby woke up and saw him, he would be reassured. Worked like a dream. Don't know whether she got lucky or whether it's worth a try? (I guess at this point, anything is worth a try ?) Although - I guess you may have to wait a little on this one - not sure a 17 month old would understand - and at 4 weeks, I'm fairly sure the second one isn't sleeping through the night as yet - therefore, waking patterns could be a nightmare !