Warning! Think twice (or four times) before trying this!

I also have a Nuvi 40 with broken USB connector and tried to update my nuvi 40's fimrware to 4.00 via SD card as suggested above. On reboot I had lost everything stored on the internal drive (original maps, all voices, all languages other than American English, help information, ...). Without speech output it's essentially useless. Very sad.

I'm not sure what happened, but it looks like the internal drive is scrambled and without USB connectivity I fear it will be impossible to recover.

Carefully consider whether the risk of losing everything is worth any improvements from a firmware update.