Galatians 4:10 and 4:21-31 are referring to separate topics. Regarding Galatians 4:10, the previous two verses clearly show that these days were not biblical days of worship, but days they observed prior to coming to the knowledge of Yahweh.
“Howbeit then, when ye knew not Yahweh, ye did service unto them which by nature are no mighty ones. But now, after that ye have known Elohim, or rather are known of Elohim, how turn ye again to the weak and beggarly elements, whereunto ye desire again to be in bondage?”
Paul states here that the Galatians were doing service, i.e., worshiping, those who were “no mighty ones.” This is not referring to Yahweh or to His worship. Many historians trace the Galatians to the ancient Gauls, a Celtic people from northern Europe. The Celtic religion was especially pagan. For example, Halloween derives from the Celtic observance of Samhain, a day connected to human sacrifice.
Regarding verses 21-31, Paul is showing a distinction between the two covenants, i.e., the covenant of bondage, representing the sacrificial laws, and the covenant of promise, representing salvation through the blood of Yahshua the Messiah. Therefore, Paul’s reference to being “under the law” is limited to the sacrificial system. Hebrews 9-10 confirms that Yahshua’s coming did not annul the entire law, but only those aspects dealing with the sacrifices.
It’s important to realize that Paul was battling two fronts within Galatians. He was confronting the Galatians on forsaking biblical truth for their previous pagan worship. He was also combating those Jews who were trying to convince the Galatians that salvation could only be obtained through the Old Testament sacrificial system. We see examples of both of these conflicts within the fourth chapter of Galatians.