The 4MHz would make a nice intermediate frequency, with a cheap ladder filter. Tweaking QRG is hence restricted to the 3.5x MHz frequency
A couple of options
- pull 80m xtal down
- pen the 80m xtal down (super VXO)
Expanded Spectrum Systems sells 40m crystals, which, divided by 2, would result in in-band frequencies.
- 7.042 MHz => 479kHz
- 7.050 MHz => 475kHz
- 7.055 MHz => 472.5kHz
For a WSPR transceiver, the 7.050 MHz option seems ideal. A small downward pull of some hundred Hz should be easily doable without compromising stability. Mind you, WSPR need a "USB dial frequency" of 474.2kHz.