Dustin Smith has a useful video addressing Heb 6:4-6:
Why the Warning in Hebrews 6 Must Be Heeded
More on the "warning passages" in Hebrews:
James White (and John Owen) on Hebrews 10:29
Two Arminian Views on Hebrews 6 and the Apostasy of True Christians
Two Arminian Views on Hebrews 6 and the Apostasy of True Christians