Mark Goodstein of X1 today announced the version 5.0 (Build 1500zzb) of X1 in his letter to all "X1 customers and fans". The best part is that it's free to all X1 users. You will need to re-index, from scratch, so don't upgrade unless you don't need to search for a little while.
It now sports a new, more standard, interface; an integrated index, allowing a single search across all of your data; huge performance improvements, which means you'll notice it less when you're not using it, while getting your results faster when you do use it; no limits on the amount of data that can be indexed (there used to be a 4GB limit to the size of our index files...no longer); tighter integration with Outlook; support for Thunderbird (and coming support for Lotus Notes); and many more things.
The current public release (build 1500zzb version 05.00) is now the latest version. It can be downloaded here or here
I have used the beta version of X1 5.0 extensively and find that it is vastly improved over version 4.0. The interface looks sleek, indexing is very fast, supports lot of file types with built-in preview. It is definitely a cut above the rest.