Copy/paste/modify error for xperm4 implementation (from xperm8)

[ssecatch-w]

This code appears to be copy and modified from xperm8. The bus ripping of [i+3..i] appears to be correctly modified, but the offset calculation of 8index was not modifed to 4index.
result[i+3..i] = if 8index < sizeof(xlen)
then rs1_val[8index+3..8*index]
else zeros()

Note that there is a bounds check issue in the specific of missing the -1 for the end as reported in this issue:
riscv/riscv-crypto#202
The bounds in the SAIL model already appears to be correct

[jrtc27]

Duplicates #171 #186