When a human character gets 4 bonus skill levels at 1st level and 1 point per level thereafter, you do NOT multiply that initial 4 bonus points. The D&D 3e (both 3.0 and 3.5) Player's Handbook is explicit about this. I don't know of Sláine is similarly explicit because the text may have been cut from the SRD on which the d20 Sláine was based. But the intent is the same - humans can maintain 1 additional class skill at full investment over non-human characters. That translates to 4 skill points at 1st level, +1 skill point per level after that. You do not multiply that 4 skill points further because it has already been calculated into the statement.
So, if the witch gets 4 skill points/level (16 at 1st, 4 per level thereafter), is human and so gets 1 skill point/level (4 at 1st, 1 per level thereafter), and has an intelligence of 18 for 4 skill points/level (16 at 1st, 4 per level thereafter), she should start with 36 skill points. She will then add 9 skill points per level thereafter (4 for witch, 1 for human, 4 for intelligence).
Any character's starting skill points will always be 4x what they get per level thereafter. So your calculation of 9 skill points per level in the post above mine is right - and tells you that she gets 36 skill points at 1st level.