Hello, I hope I can be of assistance.
From the description you've given this is likely to be due to an engine misfire.
The most common cause of a misfire on this engine is an ignition system failure, usually the ignition coil, a spark plug or HT lead.
If possible check the output of the ignition coil, it's usually cylinder 4 that fails for some reason, if there is no output then check the coil is being switched by the engine management, the middle wire of the small 3 pin connector on the coil should be 12 volts with the engine running, the 2 outer wires should be switching to earth, if they are but there is no HT output then replace the coil.
The spark plugs on this engine can become corroded due to water ingress either due to corroded cylinder head core plugs or water getting through the washer jets and dropping onto the engine, if the plugs are corroded then check for this.
There are companies that sell the full ignition set at a reasonable price such as http://www.ebay.co.uk/itm/FORD-FOCUS-1-4-1-6-IGNITION-LEADS-SET-COIL-PACK-PLUGS-/280404242231?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item4149652f37
usually if this problem persists the engine management light will come on or flash.
Start with checking this, let me know if I can assist further and I'll reply as soon as I'm able to.