120mm is about right for splitting (or sawing) into 2. Probably won't get 4 staves.
Split in 2, paint the ends with PVA or old paint, this stops the ends drying too quickly and splitting (checking). Leave for 6-9 months then de bark, then leave until they've had a year then they are seasoned.
You can also de-bark immediately as it may come off the green wood easier, but some woods can dry too quickly (probably ok with Elm)
If you want to get on with making a bow, then take one stave de-bark and work it green, but don't flex it too much.
You can clamp it to a former while green and it will dry out taking the shape of the former (E.G reflexed tips, or deflexed at the centre reflexed at the tips).
As a rule of thumb they say a year per inch thickness, so if it is worked down to bow dimensions it will season in a few months.
You can speed it further by putting it somewhere warm but there is a risk of splitting.
See my website (google delsbows) The Hazel bows section has one worked green and one done in about a month.
What sort of bow are you aiming for
longbow or primitive flatbow. I'd think Elm would make a lovely primitive.
Del