  2+1d topological phases are well characterized by the fusion rules and
braiding/exchange statistics of fractional point excitations. In 4+1d, some
topological phases contain only fractional loop excitations. What kind of loop
statistics exist? We study the 4+1d gauge theory with 2-form $\mathbb{Z}_2$
gauge field (the loop only toric code) and find that while braiding statistics
between two different types of loops can be nontrivial, the self `exchange'
statistics are all trivial. In particular, we show that the electric, magnetic,
and dyonic loop excitations in the 4+1d toric code are not distinguished by
their self-statistics. They tunnel into each other across 3+1d invertible
domain walls which in turn give explicit unitary circuits that map the loop
excitations into each other. The $SL(2,\mathbb{Z}_2)$ symmetry that permutes
the loops, however, cannot be consistently gauged and we discuss the associated
obstruction in the process. Moreover, we discuss a gapless boundary condition
dubbed the `fractional Maxwell theory' and show how it can be Higgsed into
gapped boundary conditions. We also discuss the generalization of these results
from the $\mathbb{Z}_2$ gauge group to $\mathbb{Z}_N$.
