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. 

Hi Brendan, thank you so much for your comment! I am looking to do a mix of it all (bigger cities, culture/what the locals do, outdoorsy adventure, and also the historic touristy architecture/buildings). I would like to see the bigger more well-known cities, however, I am also very interested in seeing the beautiful cities that most tourists overlook. 

I am still debating on the time I would like to travel, it would either be the summer or fall time (between July and October). I know that this is a loaded question, however, traveling to Europe from the United States is quite expensive, so I like to try and take in as much of Europe as I can. This is my third time traveling to Europe and I traveled to France, Italy, Spain, and Slovenia. I am trying to see new countries, however, if possible I would not oppose traveling to these countries again. 

And yes, I am thinking of doing a monthly continuous pass as I used it when studying abroad in Italy and it was very convenient when doing day and weekend trips! 

Keeping with your suggestion of a circle, does this sound like a good itinerary to you? And how many days do you recommend?

Fly into Croatia --> making a circle would be… 

Croatia - Slovenia (1/2 days)- Hungary - Austria (Vienna) - Germany (Munich)- Switzerland 

I would suggest to have a look what kind of good train connections are available. 

Then decide if you'd like to go always by day or night trains. For instance, in summer there are good night train connections from Split to Budapest or Vienna / Bratislava (or Prague, but this train isn't included with Eurail). Or from Rijeka to Budapest or Munich / Stuttgart.

I'd plan that way.

If budget is a bit tight, best book for September /October. Avoid mid July - August anyway, real high season. August is usually slightly better, especially at the end of the month. But prices really are lower in September - October, and the weather will usually be still summerish in September :) 

I want to see the most amount of countries during this time period.

I would ask that you seriously re-think this adventure.

Seeing as many countries in Europe as you possibly can is the absolute worst way to travel! 
For one thing it’s exhausting and secondly, you’ll not really see much! It’s also expensive to travel every day!
Try narrowing down your itinerary first and plan on spending at least TWO nights in each major stop.

When I was younger (1980s) I travelled from Dallas, Texas to the Republic of Colombia and at first it was my plan to get there as quick as I could… 
After the first couple of days travelling, I entirely changed my plan and philosophy about travelling.
I instead ‘drifted’ all the way down there and all the way back and although it took MONTHS longer it was an awesome trip. I saw so much more and experienced incredible events by slowing down my engine!

If you’re from America, and this is your first European adventure, I would steer clear of Spain and Portugal… The reason for this is that you may already be familiar with Hispanic culture and architecture and it won’t ‘dazzle’ you all that much.
I would focus on France, Germany, Switzerland and Italy. You can always come back another time and ‘do’ the other places.

Have a great trip!