I started a new job, 4 weeks ago, they have announced they have sold part of the company?

i have been told they will tranfer my employment to the company that they have sold out to, who is my previous employer.
But my job i'm leaving is being advertised, but i'm not aloud to apply for it. what rights do i have.
Answers:
I assume you are in the UK.
I'm not exactly sure the facts of your situation but I hope this general answer helps.
Under the TUPE Regulations 2006 (as updated) it would appear that you employment is being transfered to the new owners.
The Regulations operate to transfer all the contractual terms (including pay) to the new owners. Also, all liabilities (save for pension, but that's another answer) also transfer.
If you are sacked as a result of the transfer that is (almost certainly) unfair dismissal.
However, because you've been employed less than 1 year (four weeks only), you have no legal recourse in any event if they do sack you (unless it's becasue of race, sex or disability).

Further information:
Thanks for the additional information. So you've already left? If so, then there's nothing to transfer, your contract of employment has come to an end.
There's no breach of contract on the facts you have outlined.
On the further information provided, you have no basis for any claim, if that's what you're asking.

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Just fasten your seat belt and hang on.
This is why I say, "leave with class, not like an ***.,"
If you left on a handshake, you have much to worry about. Maybe they have plans to move you around, and maybe they're just advertising to see who is out there.
They have no obligation to keep you, but what you did in your old job might be the key to whether or not they keep you.
I hope they do. Believe me, you're going through a lot of BS right now, just hang tight.
I'm thinking of you.

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it would depend on the contract you have in place between yourself and your new employer and maybe the one from the old employer would play a role... i would suggest you seek some professional assistance from either a solicitor or from your countries employment board to find out what your rights are and what you can do and if you will actually need to do anything at all.