Vincent's NT Word Studies
29. We ought (dei). Stronger, we must.To obey (peiqarcein). Not often used in the New Testament to express obedience, the most common word being uJpakouw. Sometimes peiqw is used. But this word, in itself, is the only one of the several in use which expresses the conception of obedience exclusively. 'Upakouein is to obey as the result of listening to another: peiqesqai is to obey as the result of persuasion. This is the special term for the obedience which one owes to authority (arch). It occurs four times in the New Testament: Acts v. 29, 32; xxvii. 21; Tit. iii. 1; and in every case, of obedience to established authority, either of God or of magistrates. In Acts xxvii. 21, where it is used of the ship's officers hearkening to Paul's admonition not to loose from Crete, Paul speaks of his admonition as divinely inspired; compare xxvii. 10. In ch. iv. 19, Peter and John say hearken (akouein). That is a mere listening to or considering the proposition made to them. This is a deliberate course of action.
Robertson's NT Word Studies
5:29 {We must} (dei). Moral necessity left them no choice. They stood precisely where Peter and John were when before the Sanhedrin before (#Ac 4:20). {Obey} (peiqarcein). Old verb from peiqomai and arce, to obey a ruler. Only by Luke and Paul in the N.T.