You said you bought your spores from PF? Good; little margin for contaminated spores [at least the last time I bought from them].

Sterilize well, follow the substrate recipes to the letter.

Flame or otherwise sterilize your needle before each jar.

Keep your jar around 80 degrees if at all possible; this favors faster growth/outcompeting any contaminants.

Wait a few days after complete colonization before birthing. You might even wait until you see a few pins in your jars.

With only four cakes, you might get better results and easier prep by using a small styrofoam cooler. I like to cut a rectangle out of the top and glue plastic wrap. Now you can put the lid on the cooler and it has a nice little window! This also makes for 'overhead lighting only'; some aquarium setups end up with shrooms tryin' to bloom out of the sides. Also, most of these coolers are wedge shaped, so you use less perlite. It's a mess, you'll understand when you have to clean it up.

Perlite; Make sure the water level is only about half that of the perlite level. Common mistake.

Extra safety; On your birthday, try to be extra clean--lysol, bleach water, hepa, close the windows--and after closing up your aquarium/cooler, don't fan until you see pins. Usually it doesn't matter, as healthy mycelium can ward of many invaders. The shock of being 'birthed' is more than enough of a CO2-fruiting trigger for your cakes. After pinning, fan at least a couple times a day; Depending on your setup, misting may not be necessary. Another reason I suggested the cooler; smaller volume, quicker rises in humidity after fanning, usually no misting necessary! I have had to mist every aquarium to keep above 90% with regular fanning.

Good Luck!