Indeed 4X5 printed to 8X10 can be about as stunning as a contact print if the original negative was critically sharp. I think the resolution of the negative really drives the visual impact. The quality of the taking lens and the enlarging lens is most important.
I think understanding this is fairly simple. If you are really fussy about your 4X5 negative you'll pick a hyperfocal distance based on a COC of say 20 or 30 Ám for example. If you are successful in recording this level of detail on film then a 2X enlargement of a 4X5 will yield a 40 to 60 Ám COC on paper - easily beyond the resolution of the eye with a view distance of a foot or so. For 35 mm originals at the same COC enlarged to 8X one has a COC on paper of 160 to 240 Ám which will show a bit of fuzziness at a foot viewing distance.
OTOH a very high quality 35 mm lens might be able to achieve a 10 Ám COC (at least over a flat field) which at an 8X enlargement would yield an 80 Ám COC on print and be about critically sharp at a foot viewing distance.
It's just a matter of how sharp the negative is and the degree of enlargement. Actually one needs to apply a Nyquist limit to the enlarger lens such that the resolution on paper is degraded more than my figures above by Rt = 1/Rl + 1/Re. And in addition some papers are inherently low resolution due to different surface textures. Well, there are numerous variables here.
Nate Potter, Austin TX.