Diagnosed with 4mm umbilical hernia & sliding hiatal hernia. Is there any risk to have the operation for both same time three months later? Or urgent?

Either or. first there is no urgency to have an operation for either. the treatment for hiatal hernia is not surgery. 90-95% of patient with hiatal hernia do not need or required surgery, treatment is medical,mostly we treat the heartburn. the other reason is very few qualify medically to have the surgery,you have to pass many motility test to be qualify otherwise the surgery will do more harm small umbilica.
Neither condition. sounds like emergency. Consult with a surgeon, but a 4mm umbilical hernia can be repaired at the same time as your hiatal hernia, if this needs operative correction.