1kg/2.2lbs Aluminium Backplate with a 11.3kg/25lbs travel single tank wing for 11L/AL80 cylinder. I wear a 0.5 mil full skinsuit with a 3mil hooded vest and battery heated cummerbund as needed for diving in the tropics. I don't use any additional lead weight. 168cm tall and weigh 70kg.
Do not use a single tank adapter (STA): it places the AL80 cylinder too high on your back such that the first stage of your Reg will keep butting the back of your head as you tilt your head up to look at that Manta Ray flying above you. Just use tank cam straps threaded through the BP slots, and a pair of bolt & wing nut fasteners to fix the wing to the BP.
That's just not true. The STA doesn't place the tank too high, rather it needs to be high enough so that you can safely manipulate the valve, most people have their tanks so low you can't actually reach it in an emergency. There is a nice happy medium in height, and the STA sets the tank far enough off of your back that you can keep it higher without hitting your head. It also adds 2lbs to your rig which is nice.
https://fbcdn-sphotos-h-a.akamaihd...._=1417921023_f959e3e5c88caba700593b4c15b9a537
Note the picture there which isn't quite as clear as I'd like, but it's a student on their OW Checkout dive. SS BP/W with STA, tank is at appropriate height and they have their head back.
https://fbcdn-sphotos-g-a.akamaihd...._=1418793308_af60073814bdbb11733a94c86630e93a
fellow instructor and you can see here that his head actually goes underneath most of the hoses. This is with a poseidon first stage, so nice and small, the big turret first stages are a little harder to work around because they're so tall, but doable. There is a fine balance on where the tank goes, and it usually is the top cam band on the STA sits just below the crown of the tank, which is also about where if you hold the shoulder straps up, they are inline with the valve outlet.
If you only need 4lbs, I would get the aluminum backplate, STA, and SS cam bands, that will give you right around 6lbs of ballast total which is just over what is needed to sink the AL80. If you expect to be wearing any sort of exposure protection and using an AL80, going to SS isn't going to hurt you. You'll be a little heavy when you're in a bathing suit, but nothing you can't safely kick back to the surface.
We require students to be able to pull a 10lb diving brick from the bottom and conveniently a full AL80 with a SS BP/W just so happens to be right around 12lbs with regulators and the odds of a total wing failure are pretty slip. Worst case you can usually clip it upside-down or on it's side and hold onto it in the event of a tear or inflator/dump failure. You need 4lbs of ballast with most aluminum tanks right of the bat, and if you're in shape that is usually all you need.
Your rig needs to be balanced with nonditachable weight to where in a bathing suit, you need no extra weight and are not using any air in your wing to mostly hover. Unfortunately this is damn near impossible to actually do, so you're usually 1-2lbs negative. The ditchable weight is your depth compensation for exposure protection. I.e. if you're 5mm wetsuit is 10lbs positive at the surface and you're diving to 100ft, you're now going to be 8lbs negative at the bottom, so the 8-10lbs of ditchable weight that you have to compensate for that exposure protection should be ditchable, so you're not kicking it up. There is a fine balance for this because in reality you only want maybe 4lbs of that to come off at a time depending on what depth you're at so you don't skyrocket to the surface in an emergency. Being 1-3lbs negative in a bathing suit is not going to kill you, and if you plan on being in a wetsuit for any of these dives, the extra weight of the SS plate is probably better. It can be taken as carryon anyway, and a fully kitted out single tank BP/W setup is between 10-12lbs, and most travel rigs are 6-8 ish, so you're not carrying a whole lot extra for the convenience of not having to use weight.