...but our yard is "new" and is in much need of improvement. Our plan is to use the tractor for a year or two, and once the property is in order go back to a larger ztr for the mowing...
You are exactly right. A ztr is hands-down the fastest, easiest mowing machine on a smooth, finished lawn. But there are millions of acres across America that aren't quite so smooth and finished but still need regular mowing, where a ztr can't do its quick zip 'n clip routine. I've got a few acres like that, apparently so do you. A cut or scut eats those acres up, albeit at a more, shall we say, dignified pace.
As for your situation, mowing 4 acres with a 54" belly mower is a big job. I've done it, and it takes me at least 6 hours, much of it trimming. It sounds like your options are somewhat limited. Going to a 60" deck would help, but probably not a real game changer... it will still be a lot of mowing. If your 4 acres is open enough, a 3 pt finish mower could work, although it's not going to be very handy if you have lots of trimming to do. A 3 pt mower does have the advantage of being a more marketable item if you decide to sell it later when your lawn improvements are done and the ztr takes over again.