Super Tigre had some 4-bolt heads on .60s many years ago. the reason they worked and also the reason the AX should be OK is the cylinder and head are designed differently than for a 6-bolt head. The cylinder extends up above the block/crankcase and the head is grooved or machined out to fit down over this. If you take the head off an FX and an AX you'll immediately see the difference.

The reason for the 4-bolt head is the wider front transfer port passage which would have not been possible with a 6-bolt head.

The plastic needle valve assenbly seems to be air tight, at least. The fuel nipple is also 90 degrees from the needle, making it easier to hook up the fuel line after re-fueling. The plastic needle assembly can also be replaced. The cast-on FX backplate/needle assembly required you to buy a whole new backplate. The AX one can also be rotated to different positions and screwed to the firewall.

Finally, the lower portion of the AX needle assembly is relieved so you have a straight shot at the upper right engine mount bolt. I consider that great enginering from a user standpoint.