A "rule of thumb" can be taken quite literally when it comes to estimation. The distance from the tip of the thumb to the first knuckle is a crude estimate of about one inch. Likewise, the distance from the elbow to the wrist covers roughly a foot. Children are good at making associations and although they might not be able to guess how far 100 yards is, they can usually estimate a football field's length, which can immediately be translated into roughly 100 yards.
One of the key areas of estimation that will last fourth-graders a lifetime is numbers. Numbers can quickly become difficult to add, subtract, multiply and divide -- at least if you haven't learned to rules of rounding -- which creates a quick estimation. The general rule for rounding is making any number not ending in zero "round up" or "round down" to the nearest number ending in zero. This means that 12 becomes 10 and 87 becomes 90. Numbers ending in "5" are rounded up. These rules make adding and subtracting quicker, as students eliminate the last number. Multiplying also becomes much more convenient as all that is required is to multiply the numbers not ending in zeros, followed by adding the leftover zeros to the end of your result.
Related to numbers, time is rarely told accurately; rather, for efficiency sake, time-telling generally relies on a close approximation. Key phrases are associated with certain times during the day. One quick rule is teaching kids the phrases "a quarter until," "a quarter past" and "half past" -- each of which signifies either 15, 30 or 45 minutes into the hour. Another tool for numbers that fall in between these time slots is to tell children to round to the nearest five-minute hand.
Liquids can also be measured and estimated based on familiar objects. Most small drinking cups measure roughly 1 cup, which can also be an estimate for ounces (with 1 cup equaling 8 oz.). Students can then estimate how many cups make up a liquid amount, all of which can be quickly translated into ounces -- 16, 24, 32. Large soda bottles also make good estimate points; most are 1 or 2 liters, which equates to either 34 or 68 oz.