Can I submit/ArXiv without my advisor? I am being blocked from publishing my work
I suggest you respond to the student and your former advisor roughly as follows:


You do not consent to their proposal to publish the paper, as prepared, with you listed as 4th author. You believe that the substance of your contribution entitles you to first authorship, either on this paper, or on another paper to be published with citation priority.
You understand that your decision to withhold consent blocks the publication of the current work. 
You would welcome an amicable resolution to the impasse that meets the needs of all parties.
Your proposal is [** what you said above **].
You welcome any alternative proposals from them for how to divide the work, in ways that are fair, and consistent with previous understandings and contributions. 
You propose that if you cannot between yourselves craft a mutually acceptable solution within a reasonable timeframe -- say, 30 days -- then the matter should be submitted to the University Ombudsperson with a request to help mediate the dispute. 
** [Edited:] If asked: You are open to alternative proposals for referees, but you want to keep the Ombudperson involved as the core arbitrator. (The reason: Other mediators might be more knowledgable about the professional and intellectual details of the case, and thus might be genuinely helpful. But they might be your ex-advisors allies, and/or craven cowards who just want problems to go away.) 


I would cc everybody concerned on your letter, including the office of the University Ombudsperson. This last cc gives the Ombudsperson a 'heads up', and also puts a time-stamp on your letter with a (we hope) neutral third party. It also serves as a 'cease and desist' demand temporarily blocking any attempt to submit the paper for publication without your consent.

I am sorry you find yourself in this difficult situation.