I could do the same thing you have here (assume Peter meant Sabbath day) when the text doesn't say Sabbath day.
Here goes:
The weekly Lord's Day is a shadow of the 1000 year long Lord's Day to come.
As Peter explains - 2 Peter 3:8
See anything wrong with that exegesis? You should. It's called isegesis. Once again, this text isn't talking about The weekly Sabbath day or the weekly Lord's day, or Peter would have said 'to the Lord a Sabbath Day is as 1,000 years'. But he didn't. All days of the week (not just the first or the seventh) are as to the Lord 1,000 years are to us. That's what Peter meant.
Finally, a passage that speaks of the weekly sabbath.
Umm, that's my point. The substance of the weekly Sabbath is Christ the Lord.
Colossians 2:6-7, 9-12, 16-17 Therefore as you have received Christ Jesus the Lord, live in him, firmly rooted and built up in him and established in the faith, just as you were taught, abounding with thankfulness. because in him all the fullness of deity dwells bodily, and you are filled in him, who is the head over every ruler and authority, in whom also you were circumcised with a circumcision not made by hands, by the removal of the body of the flesh, by the circumcision of Christ, having been buried with him in baptism, in which also you were raised together with him through faith in the working of God, who raised him from the dead. Therefore do not let anyone judge you with reference to eating or drinking or participation in a feast or a new moon or a Sabbath, which are a shadow of what is to come, but the reality is Christ.
See, the reality of the weekly Sabbath is Christ. Since Paul received Christ, was buried with him, and raised together with him in faith, he is now firmly rooted and established in Christ (the very substance of the shadow that was the weekly Sabbath). This is one of the other passages besides Heb 4:3 that provides justification for saying Paul, in reality, is observing the Lord's Sabbath rest right now.
Simply put, you err here. The tense of the verb enter is present tense, not future tense as you say. It is inspired so, present tense, for a reason. Your "enter that rest [future tense]" is just flat out wrong and therfore unbiblical. Go check it out: Test what I'm saying about it.
http://biblehub.com/interlinear/hebrews/4.htm
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It's a present tense verb, "we enter".