4th semester results of GU Board,  will be declared on September and last date of CAT's online application form is also on September . So if the 4th semester result date exceeds CAT's last date of form fillup, i won't be able to register for CAT . So what alternate solution can i expect for this problem? 

So you should apply for Cat then ,  you will have a option in application form that are you in final year of Bachelor's degree . Usually your results will come before last date for cat application form because they also know that students may apply for entrance exams for further studies. But if it exceeds then too you should apply for Cat, as your 5th semester which is your fina year will be going on right !

It is better to fill your Cat application form as you are in final year and your academics is going on .