You had an extra ) in there after your 140 before the last IF. Aligning the (...) helped to find it. I also took out the extra ( that you have before all the IFs.
Code:
=IF(
OR(I4="Hatfield", I4="Huntingdon", I4="Letchworth"),
90,
IF(
OR(I4="Bedford", I4="Flitwick", I4="Stevenage"),
125,
IF(
OR(I4="Downham Market", I4="Kings Lynn"),
70,
IF(
OR(I4="Luton", I4="LAP", I4="Luton Airport Parkway", I4="St Albans", I4="St Albans & Harpenden"),
140,
IF(
I4="Hitchin",
100,
"INVALID VALUE"
)
)
)
)
)