I figured mine out based on my body fat percentage. In January, I weighed 170 lbs and was at 40% bf, according to my handy scale that tells me these things. (They are relatively inexpensive, and accurate within an individual - ie, they might not be objectively entirely accurate, but if it says you've lost 5% bf, it's probably right. The more evenly distributed your fat is, the more accurate they are objectively.)
So, if 40% of my body is fat, the remaining 60% is lean mass, which I want to keep. By doing lots of muscle-building exercise and eating in a way that allows me to lose mass overall, I avoid the tendency to lose muscle as well as fat (if you diet alone or just do cardio work, between 10 and 50% of your weight loss is lean mass, not fat.) So because I happen to like lifting weights, keeping that lean mass (which works out to about 98 lbs) is reasonable. My goal is 25% body fat, which is healthy but not too thin, which would put my goal weight at approximately 130 lbs.
Right now I'm at 150 lbs and 35% bf - which still works out to around 98 lbs lean mass, so I'm right on target.
In a nutshell - find out your current body fat, calculate how much lean mass you have, decide on a good body fat percentage goal (generally 20-30% is healthy, for women), factor in your weight loss style (are you likely to maintain or lose lean body mass?) and go from there. It's more complicated than just picking a number, but it's also likely to give you a more realistic goal.
Also - I found that I have much more lean mass than I did pre-baby - children make muscle! Especially if you carry them a lot. Pre-baby I weighed around 127 but my body fat was over 30% - so only 87 lbs lean body mass. And I worked out a lot then, too. So it's entirely reasonable that after 4 babies you will weigh more, but it's not necessarily a bad thing.