Or HaChaim on Numbers 4:2/אור החיים על במדבר ד׳:ב׳

1 נשא את ראש בני קהת, "take the sum of the members of the Kehatites, etc." We have already explained in Parshat Ki Tissa that the act of elevating the Israelites, i.e. numbering them (Exodus 30,12) was performed through their handing over the half shekel which formed their ransom money for the sin of the golden calf and that this is the reason the Torah used the term כי תשא את ראש when you "lift the head" when describing their being counted. This is in contrast with the members of the tribe of Levi who had no need to pay a ransom for their soul seeing they had not been guilty of that sin. This is why the Torah introduced the instruction to count the Levites (3,15) with the words: פקד את בני לוי, "count the members of the tribe of Levi." If all this is correct, why did the Torah change its wording when it came to counting the Kehatites and employ the same term נשא when instructing Moses and Aaron to count them?

2Perhaps the fact that the Kehatites were entrusted with a task such as carrying the Holy Ark and the Table which required them to enter the Tabernacle was a special elevation for them and this is why the Torah wanted us to know this and wrote the term נשא. It is a relative term and shows that their function was more highly rated than that of the family of the clan of Gershon although Gershon was the older of Levi's sons. The reason that G'd chose the Kehatites for this task was that they provided "light for the world" in that Moses and Aaron were descended from them. It was no more than fair that the branch of the Levites who had produced Moses and Aaron should be the ones entrusted with carrying the Torah which Moses had communicated to the people.

3Our verse is careful to say מתוך בני ישראל, "from the midst of the children of Israel," seeing that Kehat was the middle son of Levi's three sons Gershon, Kehat and Merari.