I have rapid shot and manyshot and am level six. Do i get four shots?
Technically no.
On a full round action you get many shot and rapid shot, as well as your 2 attacks from having a bab of +6. Assuming that you have that bab +6 you will fire 4 arrows, over 3 attacks.
Our first shot will be 2 arrows. You roll once for the attack, if it hits, they both hit. If its a critical hit, you only do it for one arrow. The next 2 attacks, are your 2nd hit from the higher bab and then the rapid shot arrow.
So 4 arrows, 3 attacks. 2/1/1= arrows fired.
Grizzly is correct. To illustrate it in a different way:
Normal attacks:
+6/+1, two attacks.
With Rapid Shot:
+4/+4/-1, three attacks.
With Rapid Shot and Manyshot:
+4*/+4/-1, three attacks, *where the first attack is two arrows.