I have worked there for a little over 4 months. i have only singed a contract for relief work and the contract available is not suitable.
Assuming you have been employed as a relief worker under a relief contract then that is what your contracted position would be. If the employer wants to change this contract they would either need to gain your consent or they could try forcing the changes through or terminating your current contract and offering you a new one on the new terms.
Whist workers will generally have some protection against this, such rights only really kick in once you have had 2 years' continuous service with that employer. That is because if you have been continuously employed at your place of work for less than 2 years then your employment rights will be somewhat limited. Most importantly, you will not be protected against unfair dismissal. This means that your employer can dismiss you for more or less any reason, and without following a fair procedure, as long as their decision is not based on discriminatory grounds (i.e. because of gender, race, religion, age, a disability, sexual orientation, etc.) or because you were trying to assert any of your statutory rights (e.g. requesting maternity/paternity leave, etc.).
So it means that if you do not sign the new contract, they can very easily terminate your employment by simply giving you the required notice period under your current contract and you will not be able to challenge that. It also means they can end your current employment and re-employ you immediately on the new contract - if you do not like that then you may leave but again you will not be able to challenge the forced resignation as you would not have the required service.
I am sorry but I am not satisfied with this answer and would like to get a second opinion.