Negotiating Salary as an External Candidate

I've applied to two jobs in the OPS and I'm currently employed by the City of Hamilton. I don't quite understand how salary negotiations work with the OPS. I've been offered an 04 position but I'm also awaiting an offer for an 06 position. I was told that upon entering the OPS , the only opportunity I'll have to negotiate my salary is when I first join as an external candidate. If I accept the 04 position for X salary, and then ask for Y salary if I get the 06 offer, will I be capped at the 04 salary because that's what I initially agreed to, or can I still negotiate a higher salary at the 6 level?