Here are the options I'd look at:
1) Drop down to 2lb fireline or stren microfuse (I very much doubt it breaks at 2lb more like 6), but it cast a lot further than 4lb fireline.
2) Fireline Exceed in 4lb. Awesome stuff it hasn't let me down.
3) Sunline Castaway 0.6PE apparently it's a 10lb IGFA line but still very thin and cast well.
One thing I'd suggest you stay away is Nanofil, although it cast extremely well overall strength and knot strength has a lot to be desired for.