Pokemon Snap Walkthrough
written by DogmaSuiae

Level 4 - River 
(Tip: Throw a pester ball at the wall with a switch towards the end of the level a Porygon will come out and hit the switch to go on to 5, the Cave)

When you first start the course there are Poliwags on a ledge to your right, use pester balls to group them together, then another to make them scurry away. On the left, are tree stumps and logs, use pester balls to get Bulbasaur to come out. There will be an area with Shellder signs, be ready to take quick pictures, because they pop in and out of the water. You'll see the top of a Vileplume in the bushes, use a pokeflute to make it dance. Next you'll see a Slowpoke, pretty dull even if you play the pokeflute. Throw pokemon food toward the brown patch near it and it will dip its tail in the water to catch a Shellder. Use pester balls to get the Metapod to fall from the ceiling. There will be a Psyduck, use the pester ball to make him jump up and down. Bigger points he you get a pic of him while he's in the air. Cloyster will fly out of the water at you right after you see the Psyduck. As stated in the tip, you will see a Porygons nose sticking out of the wall. A Pikachu makes an appearance, throw a pester ball at it and it'll run away, take pics as it dashes for bigger points.

Level 5 - Cave 
(You need 40 pokemon in your pokemon report to advance to level 6)
If you use your dash engine, you can catch up to two Zubats. If you take a picture of the first two Grimers, two more will appear later near the Bulbasaur for a better shot. If you use the pester ball 4 or 5 times on the Grimers, they'll evolve into Muk. The Bulbasaur here look strange, beady little eyes, guess what? Throw pester balls at they and they will show their true form, a Ditto. There are Koffings chasing around three Jigglypuffs throw pester balls at them to save the Jigglypuffs. Drop objects into the water and a Magikarp will flop out. The second pool of water has a Weepinbell circling around it. Throw something at the Weepinbell to make it fall into the water. It will evolve into Victreebel. You'll see a Zubat nab a pikachu and carry it, throw stuff at the Zubat to hit it, it will let go of Pikachu and the Pikachu will float down on some balloons. There are Jynx around, but they're covered over, until you play the pokeflute. By playing the flute near the Jynx, the giant egg near her will hatch and out pops an Articuno. If you saved pikachu and hatched the Articuno, turn around to face backwards, you'll see Pikachu riding on the Articuno's back. After saving the Jigglypuffs from the Koffing they will be singing at the end of the stage. if you play your pokeflute, they will get MAD!

Level 6- Valley
(Tip: This course is faster than the rest so you have to be quick with the shutter. Also near the end of the course there are Squirtles playing on the hillside near the water. They'll all leave except one of them. On top of the hill is a Mankey. Use a pesterball to hit the Squirtle while its in his shell. Aim so it will go sliding up the hill and hit the Mankey. It gets knocked down to the other side. The Mankey will be by a switch which opens up a new area.)
Right at the start there are Squirtles floating in the water, pester ball them to to get them on shore. On the walls are Geodudes throw pester balls to knock them down. By knocking the Geodude down, Sandshrews will pop out of the ground, give them food to make them happy. To see a Sandslash, you either have to use the dash engine, or get the two Geodudes to bring a Graveler down with them and it'll pop back up. There should be three Gravelers around next. When you first started the level you'll see a Magikarp near the first Mankey, throw pester balls at the Magikarp and it'll bounce off toward the Mankey who kicks it across the valley. Right next to the trio of Gravelers  you will see the same Magikarp flopping around. Throw another pester ball and it'll evolve into Gyrados. Drop object into the first pool of water near the graveler and you will see a Staryu floating above the water, take its picture and it'll follow you. Make it follow you to the whirlpool where it'll jump in and come out as a Starmie. Throw pester balls into the whirlpool and a Dragonite will come out. After you knock the Mankey down from the hill (see tip) take a picture of him because this is the closest shot you can take, then make him hit the switch he's near. Goldeens are all over the course as well are Dratini, just throw objects into the water to make them come out. THEY ARE FAST so be ready to snap off a picture.

Special Areas- The 6 pokemon symbols
(Take pictures of all these to to get to another special area where Mew hides)
Kingler: on the beach near the beginning of the course, it is a rock formation to the left as you pass Pikachu's surfboard.
Pinsir: Found near the end of the Tunnel course. You have to get the Pikachu to hatch the Zapados to turn on the electricity to the area. There will be a projector on the right showing a Pinsir shadow, this just before you see the Magnemites.
Koffing: At the start of the Volcano course you will see a volcano to the left near the Rapidashes. Throw pester balls into the volcano and a Koffing-shaped puff of smoke will rise out. 
Cubone: On the River course just above the Vileplume is a rock formation that is shaped like a Cubone. Use the poke flute to get rid of the Vileplumes' mask of smoke which is covering up the formation, by making it dance.
Mewtwo: In the Cave course just as you approach the Weepinbell you'll see glistening objects off in the distance. This is also where the set of Koffing appear with the Jigglypuff. Take a picture of the shiny objects and head to the lab. Its a Mewtwo hologram.
Dugtrio: Right at the start of the Valley course you'll see a mountain shaped like Dugtrio in the distance.

Special Area- Rainbow Clouds
(After you take the pictures of the 6 pokemon symbols this area becomes available. This is also where you can see the infamous Mew.)
Use pester balls to smash the sphere that contains Mew. It may take a few hits before letting Mew out. Use the dash engine to get closer to Mew, and throw pester balls at its head to keep in from returning to its sphere. It's possible to her 10,000 points for a perfect Mew photo.

Read about the game on the official site