can I add gps to my iPad 4 wifi only

Only WiFi+Cellular models of iPad have GNSS/GPS capabilities - this being a function of the iPad Cellular chipset. WiFi Only models of iPad have no GPS capabilities with which iPad Location Services can derive location. This fundamental difference may significantly impact your ability to use mapping or other Apps that require accurate/precise location data - in particular if real-time positioning (e.g., a moving map) is required.

WiFi Only iPad models can only “infer” their location from a database lookup of neighbouring WiFi networks for which the geographic location is known, or the geographic location of your public IP address. For static applications in urban locations, where the location of IP public IP addresses and WiFi networks are known, an “inferred” location can be relatively accurate. However, in rural locations, or when using a Cellular/Satellite/VPN connection, location accuracy of an inferred location [if it works at all] may be very inaccurate.

As is, your available options are:

• Substitute your WiFi Only iPad for a WiFi+Cellular model; this does not commit you to provisioning Cellular service for your iPad, but does provide the GNSS/GPS functionality required for accurate determination of your location.
• Add an external GPS receiver to your WiFi Only iPad. These are often used in marine, aviation and vehicular applications - and can be either directly connected via Lightning or via Bluetooth. There are various add-on receivers from third-party manufacturers - of which three possibilities are BadElf, Garmin GLO and GNS3000

No, wifi only iPads do not have the hardware for GPS.

Only cellular iPads can do GPS.