One of my twins couldnt d's or t's when she was 4 and was refferred to a speech therapist. To be honest I don't think it really helped and she is now 6 and can say them easily so maybe its just time she needs.
If you are worried it wont do any harm to speak to your hv but most schools have speech therapists working closely with teachers so if there is a problem the teachers will probably pick it up.
Also just out of interest did she have a dummy and till what age?
I only ask coz Kira had a dummy constantly till she was 3 and at night till 3.5. One of the speech therapists theory's was that she was speaking using the back of her toungue instead of the front which could be a habit from having a dummy in.