If I understand your post correctly, you are NOT looking for 4mm male-to-male and female-to-female concerters.
BTW, 4mm male-to-male (double plug) is generally not available because of safety concerns, but can be found as accessory for LV halogen lighting.
Now for male-to-female as asked in your OP. Those are used with certain coax connectors as so-called connector savers, transferring wear and tear one connector away from the instrument-mounted ones, thereby alleviating concerns about replacement cost and logevity/durability. Does not apply to 4mm connections genetrally. Also keep in mind that you would place greater strain on the original jack due to the mechanical advantage (lever). So especially a PCB-mounted jack would rather suffer more than less, except if you fixed the 'saver' to another structural part.
The second case are adaptors fo mixing safety-shrouded and regular 4mm. Safety jack accepts regular plug, so no problem. Adaptors to connect safety plugs into regular jacks are available:
Sliding shroud and naked version, which generally stays on the lead with the safety shrouded plug was made by MC/Stäubli: Stecker-Adapter LS410-I, Stecker-Adapter A4/4-Z
Versions which are fixed into the regular jack by a spreader mechanism (to convert older instrument to safety system) are also available from MC/Stäubli: A-SLK4-S, A-SLK4
They also make a 90°-adaptor for safety-shrouded 4mm (XWA-4/19) as well as an adaptor providing a bit of lateral offset (A-SLK4-RG).
http://ec.staubli.com/products/productline/6Also check Pomona and Mueller, which I haven't done.
I hope that this helps, if not, feel free to clarify your request and I'll try again. I have a reputation as being a rabid collector of T&M accessories, so the chances are good that I might know.