Vv. 1-3 Introduces the chapter context to be a call to maturity.
1Therefore let us leave the elementary teachings about Christ and go on to maturity, not laying again the foundation of repentance from dead works,
a and of faith in God,
2instruction about baptisms,
b the laying on of hands, the resurrection of the dead, and eternal judgment.
3And this we will do, if God permits.
I think the introduction and the conclusion of the chapter gives us an idea of what vv.4-6 are speaking toward.
Vv. 19-20 concludes, "
19We have this hope as an anchor for the soul, firm and secure. It enters the inner sanctuary behind the curtain,
20where Jesus our forerunner has entered on our behalf. He has become a high priest forever in the order of Melchizedek.
With that in mind, Hebrews 6:4's "who have tasted the heavenly gift" compels me to believe the writer is speaking of those who have accepted the heavenly gift of salvation.
Are you familiar with the trick to lick a cupcake to claim it as yours so that your brother wouldn't touch it?
Well, in the case of the licked cupcake, it is impossible to unlick it and so, an entirely new cupcake would have to be baked for your brother while the baker looked on sneering at you with contempt because you stole the cupcake intended for you brother... because you didn't realize that the next desert course was an entire layer cake.
So, I don't think its speaking about falling away as much as it is speaking about worrying about whether you have fallen away as far as being entirely lost. We're never entirely lost as long because we've in the second course of salvation, which is Christ's eternal hood rather than at the foundation of his sacrifice.
I hope that I was able to make sense in conveying my ideas.