Alternatives that will definitely work in this design are BC549, BC548, BC547, and they all have the same pin-out. The 'C' suffix indicates a high gain grouping. BC549 without the suffix might have either high or medium gain. It makes no significant difference in this circuit. The schematic symbol will often have a circle drawn around it, especially if it is a higher power transistor. The transistor package is described as a TO-92. In the circuit, Q4 is used as a common emitter voltage amplifier and driver for the class B output stage. It's possible to get overly interested in the gain groupings, insisting on the highest gain band possible for a particular transistor. When used as a common emitter voltage amplifier, the ultimate possible voltage gain of the circuit is much more dependent on the available supply voltage rail than the Hfe or beta value of the transistor.
BC549 used to be described as, "A BC109 in a plastic package," and such it is. BC109 is the same silicon in the older style TO-18 metal can. Plastic packages are much cheaper, though not as guaranteed to be moisture or hard vacuum proof if you're intending to send your radio into space. The plastic package is good enough for our purposes, and is easier to fit into the breadboard.
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19-APR-2026: canonicalised, direct refs