Are these joists deep enough for the joist spacing / beam spacing I am planning on my deck?
I can't find a floor joist span table that goes smaller than a 2x6. However, the American Wood Council calculator gives an allowable span for #2 Southern Douglas Fir 2x4s (~35mm x ~90mm) at 16" (405mm) centers at 5',8" (~1750mm) with a wet condition 10lb dead load and 40lb live load.  

If the dimension measurement is accurate, the joists wouldn't get past my local inspector (around here it's 2x6 minimum for deck joists), but even at 12" (300mm) centers you could probably park a small car on it with 900mm spans.