otherwise

[beachboy]

1) I'll go on studying. If I don't, I won't be able to get my certificate.
2) I'll go on studying, otherwise, I won't be able to get my certificate.
3) I'll go on studying. If I don't, I'll start another course.
4) I'll go on studying, otherwise, I'll start another course.
Is sentence number 4 acceptable? Personally, I don't like it.

[emsr2d2]

1) I'll go on studying. If I don't, I won't be able to get my certificate.
2) I'll go on studying, otherwise, I won't be able to get my certificate.
3) I'll go on studying. If I don't, I'll start another course.
4) I'll go on studying, otherwise, I'll start another course.
Is sentence number 4 acceptable? Personally, I don't like it.

In both 2 and 4, you need to follow the pattern of 1 and 3. You need a full stop after "studying" and then "Otherwise" would be the beginning of a new sentence.

I don't like sentence 4 either. It doesn't really make any sense. If you start another course, then you'll go on studying anyway. It's not an alternative, which is what "otherwise" suggests.

I plan to go on studying. Otherwise, I'll never get a job.
I plan to get a job next month. Otherwise, I might have to carry on studying.

[birdeen's call]

1) I'll go on studying. If I don't, I won't be able to get my certificate.
2) I'll go on studying, otherwise, I won't be able to get my certificate.
3) I'll go on studying. If I don't, I'll start another course.
4) I'll go on studying, otherwise, I'll start another course.
Is sentence number 4 acceptable? Personally, I don't like it.

I don't like it either. It doesn't make sense to me without context. "Otherwise" means under other circumstances. Other than what? Context could provide an answer to that but without it, the only candidate is his studying. But they will go on studying so there aren't going to be any other circumstances.

I don't know if I'm right, but I believe the punctuation is incorrect in sentence 2). I think it should be

2) I'll go on studying. Otherwise, I won't be able to get my certificate.

PS: Emsr2d2 was quicker than me. In order not to let this post be pointless, I will add a piece of context that could justify 4)

"What will you do if you pass the exam?"
"I'll go on studying. Otherwise, I'll start another course."