I'm putting a tailhook on my sterling pistol, bought an extra receiver cap and I'm going to mount a M4 adapter on the cap then a tube and tailhook. my concern being able to line up the
sights being that the tube may be too high and in the way, any suggestions?

The endcap is not a very solid mounting location - the stock will wobble like crazy, maybe even detatching the cap and disassembling the whole gun. It would be much more solid to mount directly to the receiver tube via a fabricated adapter. This would also fix the sight alignment issue you raised.

Either way, it will look like ass (no offense) - the Sterling is a classic that deserves more respect than that.

I've thought about the location already, it is a test project, that's why I got a donor receiver cap, I don't want to make any modifications to the original
equipment for resale value. As far as giving it respect, they ruined the gun by making it semi auto only. The project is intended to make it more comfortable
to shoot. As far as it looking like "ass" go bother the chuckleheads putting a shockwave blade on the VZ61 in the other forum.

Buddy, ya think you got problems ?

Try putting a brace on a Maxim 9

Funny, I just started a Mk4 tailhook project myself.