R' Samson Raphael Hirsch notes this grammatical surprise, too. He says that the initial phrase of Numbers 28:4 should not be read (as, for example, the JPS translation on Sefaria has it) "You shall offer one lamb in the morning," but rather more like "The lamb, indeed only one, you shall offer in the morning" According to R' Hirsch, the Torah is emphasizing that the morning Tamid offering consists of exactly one lamb. Unlike other offerings, in which the community is represented in its multiplicity by multiple animals, the morning Tamid is one lamb, representing the community as a united whole.
The rest of the verse explains parenthetically why, given how singular the Tamid is, two lambs were specified in the previous verse - the second one's for the afternoon Tamid, to be explained in verse 8. Elsewhere, R' Hirsch explains that the two Tamid offering brought each day represent the constant relationship between the Jewish community and God through all kinds of times. However, the emphasis here is that in each of these instances, the relationship is between one united community and God.