Peo Sjoblom wrote...
=AVERAGE(IF(A1:D1<0,A1:D1))

entered with ctrl + shift & enter
....

Normal caveats with respect to continuity - if there could be positive
and negative values, zero values should be included. If only positive
values should be included in averages, that should be made explicit,
i.e.,

=AVERAGE(IF(A1:D10,A1:D1))