I want the pass for 4 days in a month

Yes you can; the pass covers you for the majority of trains throughout Europe for the entire duration of any day on which you are using it.

But it’s worth stressing that Paris to Bruges and back in a day sounds quite full-on, so you need to think about whether it would actually be enjoyable to do this. Even if you take the fastest Thalys trains (costing an additional €25 each way), and arrive/depart as early/late as possible, you’ll get a maximum of nine hours in the city, which in practice will be a bit less once you take account of orientation and arriving at the station for your return in good time.

Bruges is a magnificent place, but it’s also the closest thing in northern Europe to Italy’s famous renaissance cities like Florence and Venice, and may have queues to match for some popular attractions i.e. the Basilica of the Holy Blood. To my mind it’s a place worth spending a night in so you have the opportunity to see it when it’s a bit quieter and have a more relaxed and potentially less crammed visit, but that’s just my personal preference.

If, on the other hand, you just really want to see Bruges, you can’t spare more than one day and you don’t know if you’ll get the opportunity to return to Europe for many years, maybe it is worth it after all. The main thing is that it is possible, so it’s entirely up to you :-)

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P.S. The two options for getting there/back are as follows (on both routes only the initial trains from Paris require seat reservation):

Fast and expensive:

Paris Nord < > Brussels South // Thalys // €25

Brussels South < > Bruges // Intercity // €0

A bit slower but cheaper:

Paris Nord < > Lille Flandres // TGV // €10 but book early otherwise €20

Lille Flandres < > Kortrijk // Intercity // €0

Kortrijk < > Bruges // Intercity // €0

*Please note that language areas are taken very seriously in Belgium, so the names of stations displayed on destination boards will vary depending on where you are.

When you are in Paris, Brussels South will be displayed Bruxelles-Midi.

When you are in Brussels, Bruges will also be displayed Brugge.

When you are in Bruges, Brussels South will be displayed Brussel Zuid.

When you are in Lille, Kortrijk will be displayed Courtrai.

When you are in Kortrijk, Bruges will only be displayed Brugge and Lille will be displayed Rijsel.

Note that Brugge is often said to be overrated and nearby GENT=a bit easier to reach a=in shoeter time, as has much the same canalside views of medieval houses and a studentsy urban atmosphere. On 1st  SUNs of the month local trams are free and shops open. In that case go via Rijssel and stay on train (running on to Antwepen)