Hey Pocketmego!
Trash has various release schedules depending on the country in which you live. I visited their Facebook page this week and found someone who said series 1 is just being released in Egypt! So, as you can see, it all depends.
Series 4 JUST came out here in the U.S. about 2 weeks ago as Jeremy0477 stated, and you can find them at your local Toys 'r Us. If you have not found them yet, don't worry, you will be able to soon as not all stores may have received their stock yet.
Series 5 was released in Australia as series 4 is being released here; Australia gets each series first. That is the reason why you can find series 5 on eBay.
Please check out this website. It has a lot of great information regarding Trash Pack, series, exclusives, etc.: http://thetrashpack.wikia.com
Hope all of this helps!