NIH is required by Congressional mandate to keep the average research project grant (RPG) length to 4 years. Since NIGMS primarily uses the R01 mechanism for RPGs and participates in few short-term mechanisms (such as the R21), it limits most R01 awards to 4 years.
NIGMS does award some grants for 5 years, including research program projects and centers. The Institute also funds 5-year R01s to most new and early stage investigators to provide extra time for getting their projects under way.