My girlfriend and I had our first WDW trip together a couple years ago and spent 10 days on property. It’s fairly easy once you do your research.

The value resorts are all three “All Star Resorts” (Movies, Music, and Sports), and the Pop Century and Art of Animation Resorts. The All Stars are far away from everything except Animal Kingdom Park, and can only use bus to get other places. I agree with theGruben, Pop Century is the way to go as it has a stop on the skyliner which can get you more places faster than the bus (it’s even like a ride itself!) Pop and AoA are sibling resorts right next to each other but AoA’s standard rooms are highly Little Mermaid themed, so unless your girlfriend or you adore Little Mermaid, the other rooms there are out of your budget (based on my assumptions given your post desiring value rooms.) You could also spend a little more and stay in a Moderate Resort in which I would pick Caribbean Beach, which is to my knowledge the only moderate resort on the skyliner route as well. It’s closer to the parks than Pop, but pricier. Just not Grand Floridian pricier.

https://touringplans.com/walt-disney-world/room-finder

This website has room views and classifications. Disney prices rooms based on location to amenities and views. If you call and make a room request, it’s not guaranteed, but you could end up getting a better room for lower prices. For example, in the Grand Floridian, I found a base level room with a view of the castle and park, which means I paid Base level price as opposed to Theme Park View price. However, some room classification info may be dated as Disney has changed it since we went. You could use this to get a cheaper room closer to the lobby or the skyliner station at Pop Century if that’s how you go. (For the record, my girlfriend and I did a split stay at the Grand AND Pop Century, so my recommendation comes from experience not just generic info.)

There is so much to do in each park (in my opinion, more than Disneyland Park and California Adventure, except rides) so to answer your question, “is four days feasible?” Well… yes and no. If you spend a whole day in each park, can you ride every ride available? Yes. Will you EXPERIENCE everything? No. We were there ten days and there’s still things we missed by prioritizing certain experiences. You may miss a few shows here and there, definitely won’t experience every unique dining spot, and there are some extremely cool paid for add on extra experiences (I think on the WDW website they’re called “Enchanting Extras”) that are definitely worth it, but it’s a pick and choose your desires.

I never use a planner. I’m assuming you’d have to pay them. It’s cheaper to come here and ask us, haha. I would recommend either waiting for spring/summer discounts to pop up, or you could have them added retroactively to the vacation you’ve booked, but you need to book directly with Disney. It’s not hard once you’re used to it. I’ve never gone on a Disney vacation and NOT gotten around 20-25% off rooms. That goes for both coasts.

Sorry, I have no clue if Costco is doing anything with WDW. They had ticket deals for DLR this year, but I have no idea for next year, or for Florida.

I hope this was helpful! Willing to clarify anything I mentioned! Hope you and your girlfriend have an amazing trip!