The exact same thing happened to me on Thursday. Would only select 4th gear, but would float freely between the 3rd and 4th position. No sideways movement.
However, if you select neutral (From under the bonnet), you should be able to move side to side, and move the gear stick into, say, first or second.
If that's the case, you will need the longitude cable. That's the one with the extra 'adapter bit' on the end.
The long and the short of it is you can get the cable from your local Fiat dealership, but they're not often in stock in many places, so ring around.
Also, Fiat recommend that if one goes, the other one won't be in best health, so it is advised to replace both at the same time. But at £95 each (from Fiat), it's your call!
That's a reasonable quote though. Mine cost around £100 in total. Cable ended up being about £80 I believe.
Good luck!