Q: What is forbidden in the fourth commandment?
A: The fourth commandment forbiddeth the omission or careless performance of the duties required, and the profaning the day by idleness, or doing that which is in itself sinful, or by unnecessary thoughts, words, or works, about our worldly employments or recreations.

Ezekiel 22:26. Her priests have violated my law, and have profaned mine holy things: they have put no difference between the holy and profane, neither have they shewed difference between the unclean and the clean, and have hid their eyes from my sabbaths, and I am profaned among them.
Malachi 1:13. Ye said also, Behold, what a weariness is it! and ye have snuffed at it, saith the LORD of hosts; and ye brought that which was torn, and the lame, and the sick; thus ye brought an offering: should I accept this of your hand? saith the LORD.
Amos 8:5. Saying, When will the new moon be gone, that we may sell corn? and the sabbath, that we may set forth wheat, making the ephah small, and the shekel great, and falsifying the balances by deceit?
Ezekiel 23:38. Moreover this they have done unto me: they have defiled my sanctuary in the same day, and have profaned my sabbaths.
Isaiah 58:13. If thou turn away thy foot from the sabbath, from doing thy pleasure on my holy day; and call the sabbath a delight, the holy of the LORD, honourable; and shalt honour him, not doing thine own ways, nor finding thine own pleasure, nor speaking thine own words.
Jeremiah 17:24, 27. And it shall come to pass, if ye diligently hearken unto me, saith the LORD, to bring in no burden through the gates of this city on the sabbath day, but hallow the sabbath day, to do no work therein. . . But if ye will not hearken unto me to hallow the sabbath day, and not to bear a burden, even entering in at the gates of Jerusalem on the sabbath day; then will I kindle a fire in the gates thereof, and it shall devour the palaces of Jerusalem, and it shall not be quenched.

Westminster Project - See Matthew Henry's Scripture Catechism for more info on the 4th Commandment