We don't define the fourth commandment's requirements of us by our inability to keep it any more than any of the other commandments. The follow summary comes from Nicholas Bownd’s preface to his magnum opus on the subject of the fourth commandment (see bottom for reference).
“First of all, that the observations of the Sabbath is not a bare ordinance of man, or a mere civil or ecclesiastical constitution, appointed only for polity; but an immortal commandment of almighty God, and therefore binds men’s consciences.
2. The same was given to our first parents, Adam and Eve; and so after carefully observed, both [by] them and their posterity, the holy patriarchs and Church of God, before and under the law, until the coming of Christ.
3. And it was revived in Mount Sinai, by God’s own voice to the Israelites, after they came out of Egypt, with a special note of remembrance above all the rest; and fortified with more reasons than they, and particularly applied unto all sorts of men by name; all which shows how careful the Lord was that everyone should straightly keep it.
4. The ceremonies of the law, which made a difference between Jew and Gentile, though the gospel has taken away, since the partition wall was broken down by Christ (Eph. 2:14); yet this commandment of the Sabbath abides still in its full force, as being moral and perpetual, and so binds for ever all nations and sorts of men, as before.
5. The apostles by the direction of God’s Spirit (leading them into all truth) did change that day (which before was the seventh from creation, and in remembrance of it) into the eighth; even this which we now keep in honor of the Redemption. And therefore the same day ought never to be changed, but still to be kept of all nations unto the world’s end; because we can never have the like cause or direction to change it
6. So that we are in keeping holy of a day, for the public service of the Lord, precisely bound not only to the number of seven (and it is not in our power to make choice of the sixth or eighth day); but even on this very seventh day, which we now keep, and to none other.
7. On which day we are bound straightly to rest from all the ordinary works of our calling, every man in his several vocation; because six days in the week are appointed for them, and the seventh is sanctified and separated from the others, to another end; even for the public service of God, and that by God Himself.
8. Much more, then, in it ought we to give over [relinquish] all kinds of lawful recreations and pastimes, which are less necessary than the works of our calling, and whatsoever may take up our hearts to draw them from God’s service; because this law is spiritual, and binds the whole man, as well as any other. Most of all ought we to renounce all such things, as are not lawful at any time.
9. Yet in cases of necessity God has given great liberty unto us, to do many things for the preservation and comforts not only of the beasts and dumb creatures, but especially of man. Not only when he is weak and sick, but being healthful and strong, both in the works of our callings, and also of recreations, without which necessity we are persuaded that men ought ordinarily to cease from them.
10. And herein more specially the governors of the Church and Commonwealth have great liberty above all others, who in such cases may upon this day do many things for the good of both, not only for war, but for peace; and may prescribe unto others, and the people ought therein to obey them. And as in other things they ought not busily to inquire a reason of all their commandments; so in this they ought to presume with reverence so much of their good consciences, that they know more cause of the things which they command and do, than themselves do, or is meet for them curiously to inquire.
11. The same day of rest ought ordinarily to be spent altogether in God’s service, especially in frequenting the public assemblies, where the Word of God is plainly read and purely preached, the sacraments rightly administered, and prayer made in a known tongue to the edifying of the people; where also they ought to attend upon these things from the beginning to the ending.
12. The rest of the day ought to be spent by every man himself alone, or with others (as his family or neighbors) in all private exercises of religion, whereby he may be more prepared unto, or reap greater fruit from the public exercises: as in private prayer, reading of the scriptures, singing of psalms, meditating upon, or conferring about, the Word and works of God—and that either in their houses, or abroad in the fields.
13. And as every man particularly is bound to the observation of this commandment, so more specially masters in their families, magistrates in their precincts, and princes in their realms ought to provide for this, as much as in them lies; and hereby to look to all that are committed to their charge, and to compel them at the least to the outward observation of the rest, and the sanctifying of it, as well as of any other commandment, as of not committing murder, adultery, theft, and such like.
14. Lastly, though no man can perfectly keep this commandment, either in thought, word or deed, no more than he can any other; yet this is that perfection that we must aim at; and wherein, if we fail, we must repent us, and crave pardon for Christ’s sake. For as the whole law is our schoolmaster to lead us to Christ (Gal. 3:24); so is every particular commandment, and namely this of the Sabbath. And therefore we are not to measure the length and breadth of it by the over-scant rule of our own inability, but by the perfect reed of the Temple (Ezek. 40:3); that is, by the absolute righteousness of God himself, which only can give us the full measure of it.”
From the author’s preface, Nicholas Bownd, Sabbathum Veteris et Novi Testamenti, or The True Doctrine of the Sabbath (Naphtali Press and Reformation Heritage Books, 2015) 7–9. Hardbound, dust jacket, ~600 pp. Available from Reformation Heritage Books. http://www.heritagebooks.org/brands/Bownd,-Nicholas.html