The slavery civic is your best friend in civ IV.
It allows you to sacrifice population to finish a current build, for a cost of 1 unhappiness for 10 turns. Each citizen is worth 30 hammers (except on a brand new build, where it's 20). If your build needs 50 more hammers, two citizens will be killed. The unhappiness does not increase if more than one citizen is killed. The excess 10 hammers aren't wasted either, they get carried over.
Why bother?
At low populations (say, up to 6), the amount of food needed to grow one pop is 30 on average. With a granary, it is 15. So you get a 1:2 food to hammer conversion. This is the best source of hammers early on. Even more so when you consider that usually your cities will have too much food and start becoming unhappy until you start hooking up luxuries.
At low populations, x extra food allows you to do x/2-population whip every 10 turns (roughly) without accumulating the unhappiness. It takes some work to line up builds worth more than 30 hammers if your x is 4 or 6, to coincide with when you want to whip, but it is worth it.
Most importantly, whipping gives you your settler, your library, your catapult NOW, which is much much better than LATER (especially for settlers, which convert food to hammers at a 1:1 ratio when being built, so you definitely want to whip them).
As your happiness cap increases, it is less worthwhile to whip, because #1 - the ratio is no longer 1:2 but closer to 1:1.5, and #2 - you do miss out on the yields that the killed citizens would have gotten till they grow back. Early on the cottage they might have been working is not as important as fast expansion or army or libraries, but later on it is. Also specialists become available, and are an excellent sink for extra food.
Slavery alone will drastically improve your early game if utilized properly.