I find that I never achieve 10,000 steps in a day if I use the car, or take a taxi. As part of my routine, I walk dd to and from school (rain or shine), walk to the tube and home again, fit in long walks when I have time/weather allows (for example - I walk to my local M&S once a week and that's 1.5 miles), and finally, I never take a bus when the distance to travel is direct and "walkable" (in my book, that is approximately a mile or less). I too find that I don't get much above 4,000 in a day if I don't make a huge effort.
For instance, today my total is 12,045. I walked to and from dd's school twice, walked to the supermarket and home, walked to/from/around the park with dd, and then clocked miscellaneous general strides around the flat. I made a real point of taking advantage of the fact that weather was decent today, so I could be outside/walking without being uncomfortable.
It's important that the stride is set correctly on the pedometer, and in order to ensure that I would "clock" a route on the car speedometer, and then walk it with pedometer to see if the total is rougly equal. And then if you really want to go to extremes, try the walk again with pedometer in a different position on your waistband. As I've said earlier (or on other threads), I find my totals are lower if the pedometer is too far off to the side near my hip. Mine does much better if positioned generally between the zip and first belt loop on my trousers.
I'd say not to get discouraged too easily. It definitely takes some experimenting to find out how your pedometer registers best/most accurately. And it also takes some real determination to integrate regular extra walking into your daily routine. As I final word of encouragement: I've been walking more for the past 4 months and have not changed my eating habits at all - and people are now starting to comment that I look slimmer. Those sorts of results make the effort worthwhile, and will motivate me to continue. Good luck!