Well, there are maybe easier ways …
from array import array
from ROOT import THnSparseD, TFile
nbins = array('i', [5400, 9000, 20])
xmin = array('d', [4000., 0., 1.])
xmax = array('d', [31000., 45000., 6.])
test_hist = THnSparseD('test', 'Test;XName;YName;ZName;TName', 3, nbins, xmin, xmax)
x = array('d', [4500., 10., 3.])
test_hist.Fill(x)
test_hist.GetEntries()
x = array('d', [4501., 11., 7.])
test_hist.Fill(x)
test_hist.GetEntries()
test_hist.Print('ALL')
f_out = TFile('./efficiencies.root', 'RECREATE')
test_hist.Write()