The literal translation is like this: "The sample should be measured in four small parts in turn (***4), so the approximate distribution of instruments should also be considered when testing."
"sample": it means sample, which is indisputable.
"Alternate measurement" refers to sequential measurement and alternate measurement.
"Unit": it has many meanings, generally referring to units, units and individuals. Since it belongs to equipment, it should refer to parts.
"Approximate": it means approximate and uncontroversial.
There is something wrong with the words "device" and "distribution". Device means "equipment and appliances" and distribution means "distribution". These two words are used together to indicate the arrangement and location distribution of electrical appliances. Because you didn't give the specific equipment or the full text of the manual, you can't guess according to the language environment, so you can only understand it like this.
The last word "test" is often translated into "test, check" in medical English.
Medical English is not easy to translate. This is just for reference. I hope it helps you.