The 'Mk IV' armrest from LeatherZ is a quality piece of kit, finished in leather to precisely match the interior of your Z4--mine was in beige leather with silver base. It acts both as an armrest plus additional storage--the interior storage space of 48.75 cubic inches works out at something like 5 x 3 x 3 inches. The Homecoming special price of just $125 (~£85) represented fantastic value for money.
This isn't exactly a highly technical installation--all that's involved is preparing three bits of tape and pressing hard, but seeing as it's a semi-permanent installation it's important to get it right!
Place the armrest roughly in position but upside down so you can see where the pieces of tape will attach to the existing console
Now try it right side up. Have the handbrake up to ensure it doesn't foul.
Make sure it doesn't obscure the little tray in the rear
Which on US models is used to house the BMW Assist module
Now remove the pieces of tape, say five 'Hail Marys' and press hard
The armrest doesn't obscure access to the rear console storage
And it hold the equivalent of three small digital cameras
Tim