Why not convert one of the milling machines to CNC, or is the travel insufficient? For the 4th-axis you can just use a Vertex HV6/HV4 (or similar) rotary table and add a stepper motor to it. I guess you might already have a rotary table to do them on the mill.

Most of what I said about the machine you linked to in China also applies to the Strike CNC machines, so I wont repeat it here. They've used Hiwin rails on X/Y, which is of course excellent, but the spacing of the bearing blocks on the longest axis is quite small - only the width of the gantry sides. That gets extra travel, but sacrifices so much potential rigidity. The Z-axis supported round rails (I think?) which could be fine, except not how they've mounted them. It's hard to tell from the photo so lets compare to their other machines:

http://www.ebay.co.uk/itm/4-axis-3D-...ht_1071wt_1270

That one uses profile rails which are by far the strongest type of rail, but they've put the bearing blocks on the Z-axis aluminium plate instead of putting the rails on this plate. If the rails are there then distance between the support (bearing block) and the tool tip is proportional to the axis extension, whereas the way they have it this distance is always the maximum value. Increasing this distance clearly decreases rigidity.

Also it says in their description two motors on the Y-axis, but in the picture it looks like just one motor and ballscrew? Perhaps clarify that the picture is up to date and get some more detailed ones?

The only way you're going to get a good CNC router is to, on order of cost; make it yourself, design it yourself, make what you can and pay someone to cut out parts you need, pay someone who knows what they're doing to make one to your specification. Since you already have a milling machine making one yourself seems like the best option. I just had a similar size mill to you and a micro lathe when I made mine.