Once you get to Star world 4 where are the keys to advance to other star locations? Please help!

Get to Star World 1 and get the secret exits...Get off the 5thy star road and youll be in World 7! (To get to first Star Road - go to Secret boo house in Donut Plains and beat the "Big Boo"

go to vanilla secret 1 or donut secret house.

for donut secret house:
when you see a door with yellow blocks around it have a cape and on the top right, spin jump. if you hit the middle it unleashes a vine. when you get the p switch, near the vine hit the p switch. climb the vine fast and enter the blue door.
How to beat Big boo:
Your floor are blue blocks. pick up one and hit the harmless big boo. do this 3 times and go to star road.
for vanilla secret 1:
before you see the flying koopas pickup a cape and fly to the left. enter the pipe and get to the end of that. go to star road.

I recommend vanilla secret 1.

for easy access to bowser's front door(not back sorry):
get the keys from star world 3 and 4.once you get that. warp to the fifth star.

to star world 3 key:
#1: beat lakitu and get the cloud, float up to the gray blocks and get the key on the left.
#2: you need good aiming.fly up with yoshi and when you're almost at the top, dismount yoshi and get the key.

tip for front door:
pick doors 2 and 5. they're the easiest. and prepare lots of items to defeat bowser.

there you have it.

First, after u finish world 1, u find the secret key in donut plains 1. ( Fly up 'til u see some pipes, fly up, and there is the key.) Find the key to the house. you now r in star world. find all the keys 'til you get to the portal to star world 5. go there and go to the front house.