You will have access to the BA Terraces Business Class lounge at LHR Terminal 4 when departing on a QF or BA flight from that terminal. Similarly if departing from a BA flight from LHR Terminal 1 you will have access to the BA Terraces Business Class Lounge at that terminal.
Yes, these are good lounges and worth the visit. Good shower facilities are available. Food is limited to snacks, including Walkers chips (crisps when in the UK) and a self-serve bar with good stocks.
Also note that LHR T4 has two Terraces Lounges. One is in the Lounge Pavilion near Gate 10 and the other is a stand-alone near Gate 1. The Gate 1 lounge is larger (on two levels) and if leaving at peak time you are less likely to have to wait for a shower than at the Gate 10 lounge.
I prefer the Gate 1 lounge, but others may disagree.