Pendjari sits in the far northwest of Benin and getting there is part of the experience. From Cotonou the most common route is the long drive north via Parakou and Natitingou — about 600 km and a full two days with an overnight along the way. The road is paved most of the way to Natitingou, then drops in quality on the run up to Tanguiéta.
From Natitingou it is around 75 km to Tanguiéta and then a short hop to the park gate. A 4WD is not strictly required to reach Tanguiéta but is essential once you are inside the park. Many travellers hire a vehicle with driver in Cotonou or Natitingou for the full trip — it is not cheap but it removes a lot of friction and the driver usually knows the park guides.
Crossing the border at Porga into Burkina Faso is straightforward in principle, but check the current security situation in Burkina before planning any onward overland travel — much of eastern Burkina has been off-limits since the late 2010s.