Are there similarities between the second and third seeds in the parable of the sower and Hebrews 6:4 - there is no return in any case; the person is lost.
Hebrews 6:4-6 is another one of those famous (or infamous) passages that is generally misunderstood. The key portion is the participial phrase in verse six "seeing they crucify to themselves the Son of God afresh, and put him to an open shame" (KJV). The King James, creatively ambiguous here as it often is, preserves the hint necessary to translate correctly. For this phrase hangs on the two present tense participles which are to be taken temporally = "while/as long as they are crucifying". The idea is not that the people addressed by Paul in the book of Hebrews are irretrievably lost. The problem is that these believers, after such a good start, had, over the years, fallen back into the Jewish rites still being practiced in Jerusalem pre-70 A.D. For believers in Jesus Christ, it was/is anathema to participate in the Old Testament rituals of animal sacrifice, because these rituals foreshadowed the coming sacrifice of the Messiah. Since Jesus had at that point in time already died, and His death was and is effective for our salvation once and for all, to continue to participate in these rites was to, in effect, proclaim Christ's death of no practical effect, to "crucify Him all over again", and to shame Him in the process. For if believers don't respect His death on the cross, who will? Later, Paul will encourage these wayward believers to abandon ritual Judaism entirely, to "go outside the camp, bearing the disgrace He bore" and choose the true sacrifice of praise and good works rather than the obsolete rituals of the Law which Christ has now fulfilled (Heb.12:11-15).
Paul is saying in these verses that true repentance must precede restoration. As long as one is actively involved in a sin (in this case, continuing to "pretend" through participation in animal sacrifice that Christ had yet to die for us), confession is pointless. First, one must stop sinning. Then repentance and confession will be acceptable to God. Until these Jewish believers had made up there minds to stop accommodating to their neighbors and committing such sacrilege against the Lord who had already bought them with His blood, there could be no hope of their restoration.
So this is a somewhat different thing than the "seed on the rock" or the "seed in the weeds" in the parable of the sower which you reference (on which see Peter's Epistles #12: "the Parable of the Sower"). The "seed on the rock" is definitely lost, for faith is abandoned in difficult times. The "seed in the weeds" is choked in terms of faith, and little production results (in my opinion, some of these may be saved, but one is certainly "playing with fire" to go this route). The believers to whom Hebrews was addressed were in definite danger of abandoning what they had achieved, but the whole point of the epistle to the Hebrews is to help these people cleanse their hearts from their false impressions and wrong practices, and to help them return to a fully sanctified course of spiritual growth. We will have to wait to find out how they responded, but Paul had confidence that they would take his words to heart (cf. Heb.6:9-10 directly following), something we all need to do whenever we bump into teachings from the Word of Truth which convict us of following an improper course.
Please see also the following links:
Bible Basics 4B: Soteriology: the Study of Salvation
Does Hebrews 10:26 teach loss of salvation?
Does Hebrews 10:26-35 ("deliberate sinning" etc.) mean that a believer can lose his or her salvation?
Apostasy and the Sin unto Death.
Have I committed the unpardonable sin?
Have I committed the unforgivable sin?
Messianic Legalism I
Messianic Legalism II
Messianic Legalism III
Yours in the only true Way of Righteousness, our Lord Jesus Christ,