Validation for a field to allow only decimal numbers.

Client script with fieldChanged() entry point can be used.

function fieldChanged(scriptContext) {
            try {
              let currentRec = scriptContext.currentRecord;

              if(scriptContext.fieldId =='custrecord_item_req_item_cbm'){
                let itemCbm = currentRec.getValue({
                    fieldId: 'custrecord_item_req_item_cbm'
                });

                let decimalNumber = /^\d+\.\d{0,4}$/;
                let decimalTest = decimalNumber.test(itemCbm);
                if (!decimalTest) {
                    alert('Enter decimal number having maximum of four digits after the decimal point.');
                }
            }
 } catch (e) {
                log.error('error @ fieldChanged', e);
            }
        }