Gold (brass) is a perfect choice for the dresser and will be fine with the grey headboard. I used the brushed gold on my grey kitchen cabinets and it looks fabulous.

Bronze, gold or brass all work. The dresser needs its own look. It won’t clash with the gray.

I would be more likely to find a whimsical fun knob like animals or flowers or boats depending on the child. It's a 4yr old after all.

Look on etsy for whimsical knobs

Beach Ball

FISH

You can get lots of fun knobs online. Anne at Home has some really fabulous ones and you can pick a finish. Check out my skulls!

Gold one with square knobs

Fun knobs for sure .

Those knobs not being straight would drive me crazy!! They also look like great foot holds to climb right on up the dresser.

I would either keep existing knobs or go for gold balls .

I'd keep the original knobs as round knobs don't catch on much and are comfortable for small hands. Perhaps paint them different colours? Might be a great way to use leftover (craft?) paint. Imo, would make more sense to change the hardware when the bedroom becomes teen's room.

I like how the good compliments the blue. But I thinks something a bit more.