FYI, the current 6th gen (2018) iPad is regularly available for $250. This week $250 at Target. (less 5% using a Target credit card.)
The 4th gen was a nice tablet in its time - 2012 launch. But it had an A6 CPU whilst the 6th gen has an A10 CPU. The 4th gen is maxed out at iOS 10… iOS 12 is current. The 6th gen is also the first mainstream iPad to get Apple Pencil compatibility.
Not sure how good the third party refurb 90-day warranty is, but a new iPad gets excellent support from Apple for 1-year. The 4th gen iPad works OK for now but will age fairly quickly at this point.