Many companies will specify the notice period in the contract, in this case 4 weeks, but separately note that a probationary period - 6 months say - applies, during which notice can be given by either party in accordance with the Minimum Notice & Terms of Employment Act 1993. (The statutory period is under 13 weeks, no notice. over 13 weeks, one week)
They will usually not want to tie either side in to a long notice period until they are sure it will work out.