There is no point for you to get another arranger.You still didn't mention what kind of sounds you seek.

I did think about Roland Integra 7.It has almost all of the sounds made by Roland.But, you need a keyboard too.You could get really good controller.Or, what about Korg Kronos.It has 9 different synth engines.Plus a full sampler and steaming samples up to 2 gigs I believe.I wouldn't get another Yamaha.T4 has good acoustic sounds anyway.There is also Roland BK9 with great styles and sounds too.Basically it's all up to you and which kind of sounds you want to add.There is many more choices...