You can use a 4pin EPS in 8pin, as it's not an 8pin but a 4+4pin. So it'll fit into one of the sides. The EPS is a cpu supplemental supply, much of what the cpu needs comes from the 20+4pin mains, it's mostly small OC, boost clocks, turbo power needs that come from EPS, so many cpus don't have a need for more than 4pins, only the biggest cpus and overclockable cpus do.
If your cpu will require an 8pin EPS, it is what it is and you'll need a psu to power it, regardless of cost. If your cpu only requires a 4pin, it'll work on a 4+4pin mobo just as well as a 4pin mobo.