John Gill Commentary Psalms 114:4

"The mountains skipped like rams, The little hills like lambs." — Psalms 114:4 (ASV)

The mountains skipped like rams
The mountains of Sinai and Horeb quaked and moved at the presence of the Lord, when he descended thereon to give the law; these saw his glory and trembled, (Exodus 19:18) (Psalms 68:8) (Habakkuk 3:6Habakkuk 3:10) .

And the little hills like lambs ;
very beautiful are the larger mountains of Sinai and Horeb compared to rams, and the motion of them to their skipping; and the little hills adjacent to them to lambs: these may represent the greater and lesser governors in the Roman empire at the time when such large conversions were made in it as before observed; and which skipped, and trembled, and fled, and were moved out of their places at the downfall of Paganism and progress of Christianity, (Revelation 6:14) and also may be an emblem of the difficulties which lie like mountains and hills in the way of a sinner's conversion and effectual calling, which yet give way to and are surmounted by the efficacious grace of God; all mountains become a plain before him, and when he works none can let.