If I understand you correctly, you have booked a catagory 4 cabin for two people in March 2005, but you'd like to also try a "test" cruise this July, but don't think you can swing both.
If that is the case, here's my suggestion:
Change your Cat 4 cabin to a Cat 6 cabin and do a 3 day in Cat 11 in July (or what ever level you can afford after changing from Cat 4 to 6).
From what I can tell, the only perk of the upgrade to catagory 4 is the extra space equal to a murphy bed. Based on experience, any of the Deluxe cabins is plenty of room for two on
DCL. Once you've booked at a level that gets you a verandah (starts in Cat 7 with Navigator's verandah), I wouldn't consider it splurging unless I booked a catagory 3, 2, or 1.
Starting in a Cat 11 for three days will be fun, and will make you really appreciate the verandah level next year (stated another way, once you've had a verandah, any thing less feels like your "settling").
We used to always book a Cat 6 for what became our annual DCL trip. But the year we had our first child, we had to skip. So the following year, we got the DCL itch again and decided to book a 4 day "test" cruise (to see what it was like cruising with a little one). But the only thing we could afford at the time was a Cat 11. Obviously we missed the extra room and the verandah, but we still enjoyed being back on a Disney Ship.
[EDIT]
BTW, I'm not meaning to sound like I'm contradicting Teri's statement that
We splurged once for a Navigator Verandah and were dissapointed with the room for the price difference.. I will admit that it is a very big price difference (and it's gotten worse compared to what we used to get a Cat 6 for the first several years of DCL operation). I'll even go so far as to agree that a verandah isn't worth the price difference (the last time I did price comparisons, I was finding that the verandah basically was costing $100 extra per day compared to a similar cabin without verandah) . But we've also enjoyed the Verandah's enough that we've just always decided to swallow the price difference and enjoy.