It's a horridly vague way to answer, but, "it depends". Your results will vary a lot depending on the car and the terrain.
Typically running your car in the highest possible gear in which you're not abusing the engine will give you the best constant-speed economy (without resorting to bonkers hypermiler methods such as "pulse and glide" that will raise your economy but get you killed ... by an accident, or an enraged fellow motorist), as has been proven in many tests.
The 30mph speed range is a tricky one, however, as it's more subject than e.g. 50mph to the variations in gearbox and engine characteristics - you could be well inside the economic range with an old small-engined Polo as it's best torque/best economy speed is at low rpm, and even 5th gear is shockingly low, and you're best to get into top as soon as possible (or similar for a diesel van, or the wierd little Hijet my dad used to own, which could be driven like a 3-speed using 3rd gear to pull away when you were feeling lazy).
Certainly accelerating with any kind of (relative) force at this speed won't be particularly economic unless you have quite a low-revving engine (a non-turbo diesel, for example) and a quite "short" gearbox (revving comparitively high in top gear) - with a modern car, particularly with the gearing you describe, you're going to be just nicely in the eco band in 4th, but 5th is likely to be well below it. If you're cruising, it should be ok, as the reduced restriction on the throttle and lower overall friction should balance out any creeping low-speed inefficiencies, but as soon as you want to climb a hill or gain any serious speed you'll need to downshift - or you'll soon have your foot to the floor, and the engine computer will trip into a decidedly inefficient "max power" mode where it shoves somewhat more fuel into the engine than is needed, in order to make best use of the available air... where in 4th you might not have needed to do this.
By the way, what car is that? The gearing difference between 4th and 5th seems remarkably extreme. Usually the top two gears are quite a bit closer than that (the relative difference between gears typically getting smaller as you change up), it sounds more like 1st and 2nd / 2nd and 3rd (except at much lower rpm) than anything else. Some kind of sportscar with tight gearing in the lower range and a "motorway" gear?
In your case, I'd say it would stand some experimentation, maybe drive around normally not going above 4th for a quarter tank or so (not making a special trip for this, just as part of your everyday routine), fill up, measure your mpg (miles, divided by litres, times by 4.54) .... then use 5th gear also for another quarter and measure again. Rather unscientific and imprecise, but I've used it to see the gross effects of different driving styles, gear use, etc in my own cars before. If you have a trip computer / instant MPG readout on your dashboard, use that instead.
Certainly if you bait the cameras ~34mph, or you do any significant driving on 40mph roads, 5th will be almost guaranteed to make you a small but significant saving there. At higher speeds, even more so, particularly at 55+ where your listed 4th gear will be heading decidedly out of the typical economic rpm range.
Main rule of thumb is, if it feels/sounds like the engine is struggling or you're having to use full/nearly full throttle to maintain speed, change down - it won't be doing the motor, the environment or your wallet any good (3/4 - 4/5ths throttle is ok, and about optimal when accelerating... yes, it's ok and even beneficial to have brisk acceleration (uses engine more efficiently, gets you into high gear sooner), so long as you're not over-revving or completely flooring it!). And another is to experiment yourself and find the best for your own car because I've found an AWFUL lot of crap is talked about this subject ... even by myself sometimes, so take things with a bit of salt!
NB: Diesel engines, and very modern/pricy petrol engines, are another case as well as traditional petrol ones - most diesels and some advanced (BMW/Honda, i think) petrols don't have a throttle valve, their power output is solely based on how much fuel is injected into the engine ... so they actually tend to be "most efficient" at maximum power, at least for light truck ones I've seen data for (as they're going to do a lot of max-output hauling uphill, they're designed for it... and this appears to hold true across the entire usable speed range, though it drops slightly near idle and redline). This however doesn't necessarily mean "highest MPG", but "most horsepower per fuel used on each combustion", and travelling faster/running at higher rpm/having the throttle wide open uses and produces more HP, so it uses more fuel anyways. Answer? Use highest gear and widest throttle that you can get away with, without destroying the engine :-)
Bit of a long standing interest in this myself, which is why I clicked onto it. Gone thru all the research and asking around before, found there's not much concrete data available for the common man. So sharing some of the things I've learnt or experimented with. My own car seems to run just nicely and quite efficiently at 33mph in 5th (~1333rpm), but has to be geared down for anything other than maintaining speed on the flat in this condition, and is more than likely more wasteful below this speed (where it starts to rattle, anyway!). The info on engine type/speed/throttle setting/etc economy IS out there to be found, but you're as likely to run across it at random as actually find it on purpose.
I just need to get hold of a car with a live MPG meter, or a scanguage compatible with my ancient POS Astra, so I can confirm some of the theory for myself :-)