Spot on. As you complete Security at Terminal 4 turn right and head up the escalators by Gate 43 and then follow the walkway around, Qantas First is the first lounge you meet, and is opposite the Korean (I think) lounge. The BA/CX/QF Business Lounge is next along. Allow 5 to 10 minutes each way from Gate 43.

If you clear Security at Terminal 5 look for the tunnel to T4. Important note towards the end of the tunnel, there are steps on the left leading up to T4. Look for the sign, if you miss them you end up exiting and then have to clear security again, like I once did........

I do not know about the showers at QF First but the staff are up there with the best lounge staff to be found anywhere. The food is excellent, I usually have the Salt & Pepper Calamari (its different from the Sydney Lounge) and the really good hamburger (which is way better than most eateries). Though be warned they practice the weird Aussie habit of putting beetroot on the burger.

The AA Flagship Lounge is nice, but not as spacious, and is also self service which has its advantages and disadvantages.