I read something potentially very good in 'The No Cry Sleep Solution' by Elaine Pantley. It's a very warmly written, symathetic book and the author has co-slept with all her children. There are some wonderful tips and advice and some people say it really, really works.
Perhaps you could decorate a room for them, buy them a fun new bed (all expensive I know!) and invent a night-time fairy who leaves treats or star-stickers on a chart if they stay in beds all night. The Pantley book even suggests sitting down with older children and making a 'sleep book' with them which 'tells a story' of what happens at night time from their bedtime routine onwards.
As they are 2 and 4, another idea in this book is to start a 'sibling bed' (read about these in the above book). Not a common thing in this day and age I know, but it's how most siblings slept together in our grandparents time and before.
Another method, in this book to wean a baby out of your bed, is to put two mattresses on the floor, one for you one for baby and each night, move the mattress a little bit further apart until baby gets used to sleeping apart from you. Not sure how practical that would be for two and for pregnant ladies though.
I think at their age from what I've heard, you can sort a problem like that with a good few nights determination on your part.
Good luck and I'm sure you'll have it sorted in no time. Really recommend the above book too. Wish I'd read it before I had DS and I'm certain I would have had an easier time at night with him!