A broad question. Usually we ask to specify a bit more what you'lld like to see. When would you like to travel?
Note that Eurail/interrail doesn't operate trains themselves. It's an organisation providing a nice, very flexible ticket/pass for trains of all the participating companies (95% of the trains is included in the pass). Busses, trams and metros are not included. For some ferries you can get a discount, with the pass.
What do you like? Bigger cities, nightlife, culture or more outdoorsy adventures, more laidback time in nature with plenty of hikes and such? Or a mix of it all?
Three to four weeks are not that long to get a good grasp of Europe.
First advice: you do not want to rush through. You only get to see the touristic clichés. In bigger cities or nice regions you can easily spend a small week to enjoy it all.
Second advice: best start with "easy" rail countries. Croatia is okay to do, although the network is not super big. A logical step after that is Slovenia, which has lovely nature. Then Austria and Switzerland, which both have excellent networks that are very easy to use with Eurail/Interrail. From Switzerland you can get easily to France, Germany and Italy. Large countries with plenty to see, explore and discover.
Third advice : a monthly continuous pass seems the nicest option for you.
Fourth advice: keep it geographically logical. It's always a good idea to try to make a circle, or a bit of a straight line from your start to end point during your month here.
Don't forget about seat reservations. Your pass functions as a ticket, but some trains have mandatory reservations (usually high speed trains like TGV in France, the Freccia in Italy, AVE in Spain, night trains,...). You'll be required to buy a supplement to be able to board those trains, often booked in advance. If you plan to take the Eurostar (London to Paris/Lille/Brussels/Amsterdam or the other way around) try to book it ASAP. It can sell out. Often you can avoid these supplements, by taking slower, regional trains not requiering reservations. But on Eurostar, there are no good alternatives :)
Some trains have optional reservations (mostly in Central European countries, like Germany, Switzerland, Austria, Czech Republic,...). You can reserve a seat, but you can also just board and find a seat (that's not reserved).
Seat reservations can be a bit complicated, so don't hesitate to ask advice here if it gets unclear.
Most trains have no reservations, although these are very often local/regional services.
So, if you elaborate and finetune a bit more on what you'lld like to visit, you will get some nice tips and suggestions.