Tec 9 is a good alternative. I think the Sten is only slightly better than the M4, due to the automatic feature. However, id recommend saving your 65k for something better. I recommend the Thompson or the Shipka.
Sten is good but i mean if you already have M4 maybe you should stick with it, up to you. Also depends if you dug for this money or if everyweek you have itunes card (in this case buy it). You decide.
Hai Agony
Hmm, i tried sten before myself and I feel that it is like m16 to me, probably just a bit bit faster.
If you want a fast auto gun with high damage, besides bar, Thompson is a good choice imo =)
It's funny how you all gave me suggestions on guns I already had lol. I love the Sten, it actually fires as fast as the M4, but the trade-off is trading automatic for freeze. It's an amazing gun, try it out.