System requirements: The XS-4 is a vsti instrument - it requires a vst compatible host, which runs under windows.  64bit DAWs may require bridging software. 14 MB hard disk space. (Mac version is at least 6 months away).
Version 1.200 (Build 0041 - new skin)
CPU Usage: Heavy
This is identical to the retail version except for a short audio gap which occurs once per minute.
XS-4
Click on the image to enlarge.
Visualization of the XS-4 as a physical entity. MANX
Copyright 2014 Manx Synthesizers - in association with Heliosphere Software, UK
SPECIAL INTRODUCTORY                PRICE!
( Price is likely to go up!)
$39
Approx.  £25,  €32
XS-4 Retail Edition
Click to buy & download the XS-4 retail version now. (You should try the demo version first to ensure that this software works well with your system.)
FAQ
The XS-4 is a VST host compatible virtual instrument for PC / Windows
   General Description:                              6 note polyphonic.                Three 6-stage envelope generators. Two oscillators, each can output up to 4 square                         waves simultaneously. The filter section is switchable between polyphonic                                 & mono modes. Programmable midi automation, midi learn for all              parameters, patch library.
The XS-4 attempts to reproduce the sound a group of similar analog synths which appeared in the mid 80’s. Examples are the Korg Poly 800, Siel DK-80 and Suzuki SX-500.  These instruments have a very similar architecture, but do not have identical sonic characteristics. The oscillators, typically built around chips designed to power organs, do sound very nice when run through an analog filter and a punchy set of envelopes - we felt this was a sound well worth emulating. The result is the XS-4, and were broadly happy with its performance. Regrettably, it’s rather power hungry (CPU usage) because the oscillators are outputting 8 waveforms per note and the filter / envelope stages are also now very CPU intensive.  Selecting monophnic filter mode may eleviate the issue somewhat. However, pitch modulation will place a particularly high load on the CPU, as will long loudness decay times. Despite this, we feel that sonically, the instrument has considerable merit; you can assess it yourself, within minutes, by downloading the demo from the link below.
Audio Demo - all sounds from the XS-4, (external effects applied) Please refer to the MM-2 audio demo description since this one was  produced in a similar way.