1- It is just like riding a bicycle.
2- It is just as riding a bicycle.
3- She acts just like you / his /her.
4-She acts just as you / his /her.
I have some trouble about 'just as' and 'just like'. It seems to me that sentences one and three are Ok, but I would like to ask your opinion about sentences two and four.
#1 is fine.
#2 doesn't work at all.
#3 should be 'She acts just like you/him/her.
#4 needs something to follow, such as 'She acts just as you did when you were that age.'