Not this experience, alas for you, but I accidentally dropped my GRD4 in a sandbox in mid focus while sneezing. Sand got into the lens barrel, the camera seized up. After cursing myself for not attaching the wrist lanyard, I sent it to CrisCam in Arizona. They were able to replace the affected parts for $300. They will give you an estimate before repair/billing, I think, if they have the spares.
I sold mine to recover costs & to get the GR (APSC). It may be time for a sentimental farewell and upgrade for you. The IV is harder to find than the III, so you could ever so slightly downgrade for less $$.
Robert Hill Long
Eureka California USA