The mind boggles [member=2128]Womble[/member] at you "topping" 5 acres with a tiddly mower (especially if it was a "lawn" mower!). Anything is going to be an improvement.
I'm also wondering why you are topping 5ac with a mini mower at all - you can't surely be taking hay by that method!?
Anyway, my thoughts (in no particular order):
Anything towed by your 4WD will leave corners and tight-up field margins to be dealt with by other means, but you might be happy with that - if so, the gang mower or towed power-mower would be a good option as suggested by others. Not exactly cheap as chips though, although you might (??) find 2nd-hand.
Friendly farmer neighbour for a few quid or even for free if they can cut for haylage or whatever. (Fertilizer required for good year-on-year harvest obviously - if you pay for that, then you might actually be able to ask farmer to pay you for the grass crop!!) Of course depends on how many times a year
you want a cut. If more than 2 cuts, then you will almost certainly be paying your neighbour! However, well worth considering and comparing with the capital outlay on any kind of capable meadow mower. (Field corners and tight-up-to field hedges might still need to be considered.)
Two wheeled tractor with sickle-bar cutter (as already mentioned by macgro7). This should allow you do get into the corners and up tight to hedges although you might wish to kick-through the mole-hills to flatten before a cut! (Unfortunately 2nd-hand offers for sale are pretty scarce I have found.)
Some four-legged friends
(Not quite as hassle-free as the friendly neighbour with a topper or other machinery options - unless a farmer neighbour would like a bit of extra grazing through-out the year.)