Jamieson, Fausset and Brown on Galatians 3:4

4. Have ye suffered so many things--namely, persecution from Jews and from unbelieving fellow countrymen, incited by the Jews, at the time of your conversion.

in vain--fruitlessly, needlessly, since ye might have avoided them by professing Judaism [Grotius]. Or, shall ye, by falling from grace, lose the reward promised for all your sufferings, so that they shall be "in vain" (Ga 4:11; 1Co 15:2, 17-19, 29-32; 2Th 1:5-7; 2Jo 8)?

yet--rather, "If it be really (or 'indeed') in vain" [Ellicott]. "If, as it must be, what I have said, 'in vain,' is really the fact" [Alford]. I prefer understanding it as a mitigation of the preceding words. I hope better things of you, for I trust you will return from legalism to grace; if so, as I confidently expect, you will not have "suffered so many things in vain" [Estius]. For "God has given you the Spirit and has wrought mighty works among you" (Ga 3:5; He 10:32-36) [Bengel].