Is 4-color process printing (CMYK) more or less expensive than using 4 spot colors?

Although the best answer would be ‘check some printers’, I’d say, yes, usually 4 spot colour printing is more expensive. At least for regular offset printing.

A typical printer will have their presses loaded with cyan, magenta, yellow and black ink, for this is the process that is the most commonly chosen. Changing the type of ink loaded into a press takes time and effort, and they’re prone to charge that time to you, the customer.

Moreover, Pantone inks (the ubiquitous kind of spot inks) need to be mixed. I don’t expect many printers to have all of the hundreds of PMS inks pre-mixed in stock. If you look at a Pantone colour book closely, you’ll see recipes printed below any of the swatches—that’s how the printer mixes the colour. Mixing takes time as well, and that time, you pay for.