If you already have an IPac4, then I would forgo the JPac and just rewire everything.
You don't need to use an ArcadeVGA, but you will need some method to drive the video signal to the appropriate frequency for your arcade monitor. There are some options listed here:
http://wiki.arcadecontrols.com/wiki/Arcade_MonitorsBefore you go too far, however, you should identify the monitor that you have. I'm not well-versed with fighters, but I think Tekken Tag may be a Medium Res monitor, which limits your choices some (and I'll leave that advice to folks who have worked with MAME and Medium Res monitors). It could also be that the monitor is a Standard Res monitor and that Tekken Tag has a dip switch for type of monitor (I'm too lazy to look it up right now).
I like to use the ArcadeVGA with Standard Res monitors because I have never had a problem -- and I have enough problems without worrying about video signals.