Why in the blastin' @#**!!@**heck am I having such a freakin' hard time with the fourth string???????????It's making me crazy!

I'm fine running through all the notes first string through fourth,but when it comes to reading the notation in the basic lesson songs,I get my strings goofed up..........ugh!!I keep finding the wrong string.It's my third day of doing the fourth string. Help!!!!!!!!!!!!!!!!....I only know the first three frets of the other strings....e,f,g,b,c,d,etc.Real beginner here!!