Explain to the students that there are different ways of measuring the same thing. Teach the students that in different situations we use different units as measurements. Give the students a practical example. You can try this example: we usually measure our age in years, and can compare one another's ages this way. However, the unit "years" would not be suitable for babies; in this situation we use months. Tell students that in some situations it might be useful to talk about their ages in months; for example, two students at age 10 could convert their ages to months to find who is older.
Introduce the units you wish to teach. List the units or systems on the board along with their conversions to give students something to reference.
Play a game that uses the units listed. Prepare in advance a set of flashcards that will be used in a basic matching or memory game. You can be creative with what you place on the flashcards --- the only true criterion is that each flashcard must have its matching flashcard. Examples for content on the flashcards are unit conversions (e.g. "30.5 centimeters" matches "12 inches" and "10 kilograms" matches "10,000 grams"), types of units (e.g. "feet, meters and centimeters" matches "length") and measurement tools (e.g. "scale" matches "pounds and kilograms).
Have students convert units. Hand out worksheets that have questions requesting students to convert units of specific types to other units. Students in fourth grade have the basic mathematical tools (multiplication and division) to handle such problems. An example of this type of problem is "40centimeters = ? inches."