Which crownwheel and pinion of ratio 4.375 or 4.1 will fit into my front diff housing of my Isuzu 1993. Bolt on diff. Diff at the moment is a 4.777 ratio

Are you talking about swapping the entire Diff assembly? If you are you will need to measure several things like axle width, suspension mounting positions and check to see if all the suspension mounting points match. If it bolts to the Diff it MUST line up.
Unless you have cracked or damaged the outer housing most Diffs can be completely stripped down and overhauled. You might be able to rescue the one you have.

if it is the same bolt pattern in a banjo housing
(fit in from the drive shaft end not salisbury type-- remove cover and pull crown wheel out from the rear) it will fit
items to consider are axle spline size and numbers and diff ratios
if the ratios are not the same , the swap will cause very early drive line failures

You can just change the diff crown wheel and pinion.

loosen rear wheels, jack up rear so wheels are free, fit safety stands , drain oil, remove wheels, remove 4 bolts holding axle bearings. remove both axles, remove bolts from tail shaft at diff pinion flange , leave tail shaft in transmission housing or oil will leak out, remove bolts from diff housing, the diff center can now be removed.

if there are leaf springs in the rear of both vehicles and the centre bolts of the springs are the same distance apart then it will fit unless the wheels on one are closer to the body than the other. (track) A tape measure will prove this.. The drive pinion and tail shaft u-joint set up should be the same. Check that the diff ratios are the same or you will have transmission problems ( this is the most important point). Difference between front and rear diffs.

get hold of any work shop manual that has a diff section in it and that explains how to set up the crown wheel / pinion mesh correctly. All diffs use the same settings for the positioning and pre load figures.

For the rear diff., remove the driveshaft, brake drums, diff. cover, remove pinion flange nut, move diff carrier to access side gear retainer bolt and remove bolt, slide side gear pin out and push axles in to remove axle retainers and remove axels. Mark carrier bearing retainers, spin bearing retainers to loosen carrier for removal. Remove carrier, remove pinion out of diff case, then r&r bearings. Any shims should be replaced in their original location for depth and pinion to ring contact.

Do you mean the pre loads on the pinion bearings and the crownwheel ?
Pinion is 6 - 11.5 lbs Ins on new  bearings on the big nut as you turn it up the pinion should just start to turn .
, it has a collapsible spacer ..
Crown wheel bearing caps are 80 Ft/Lbs.
Crownwheel bearings are loaded by getting the backlash to 006' - 008' thou and putting a 002' shim each side to load the tapers up .

Constant howl....usually wheel bearing.
Howl on acceleration.......usually rear pinion bearing (the bigger of the two on the pinion) in the diff (rear axle)
Howl on de-acceleration....the front (smaller) pinion bearing in the diff.(rear axle)

Pretty much a bolt out-bolt in deal...Most critical part is actually making sure that you obtain a diff with the exact gear ratio as the one you are replacing. Count teeth on ring gear and pinion gear, divide ring gear # by pinion gear # and you will get a number (like 3.23, expressed as three twenty three to one) Diffs come with a tag on diff cover but many times tag gets lost if unit is serviced.
Use flair nut wrenches on all hydraulic fittings, Plenty of wd40 on all hardware and support the body and diff while working so it does not fall on you while doing the job. Regardless t if it came with lube in it, change lube and make sure it's properly filled.
Sometimes you will need to push and pull a bit to get bolt holes to align when installing. It's easier to do if you do not tighten anything fully till every bolt has been installed.
good luck, any specific questions? just ask...