Cruising speed/revs

A quick question for those more knowledgeable  than me…
 
How many revs is too high for cruising along the highway in a standard spec 1600 Kent with a standard gearbox and a 4.11 diff?  Is it too much to be cruising along at around 100km/h with the engine sitting at 3800-4000rpm – will this eventually burn up valves/piston rings/the entire engine?  Is around 3500 much safer, or is 4000+ okay?
 
Reason I ask is that to get to work I travel on the highway for about 10kms each morning and afternoon, and don’t want to be lunching the engine for the sake of doing 500 less revs – and vice versa don’t want to be just going much slower when the engine could be quite happy revving quite high.  I also want to get into travelling further afield (100kms away) more often, and want to find out this before I start (and before I may end up stuck on the side of the road!).
 
More than happy to hear all suggestions and opinions! :)

Cheers Marty, I should have stated that I do want to put a 3.77 (or 3.54) geared diff in, and possibly a type 9 box as well, but I have to save up the $$$ first, and also find them!

In the meantime, I just want to make sure that I'm not doing damage but also not going TOO gently! :)

If you plan to put a 5 speed in leave the ratio you have as 5th will give you the relief on the highway and you will still have the better acceleration.  I have a 3.54 with a 5 speed in my RS and it is kind of pointless.  You simply cant use 5th under 100km because it cant accelerate.  I will be putting a 3.89 in that when it goes back together.
 
Also, this is just a personal peave - unless your car has an Aussie borg warner diff (e.g. plate on the rear) there is no such thing as a 4.11 ratio in an escort!  The front loaders or english axle ratio is 4.125.  Nit-picky i know, but every time i hear someone say "i have 'four-eleven' diff ratio" i really want to poke them in the eyes 3 stooges style!
:-)

Several years ago I fitted a late model 3.54:1 centre to my unmodified/factory standard 1979 built 1600 Escort panel van. All works very well including up to the 110kmh speed limits.
 
Cost was about $120 for the centre, made a gasket and found some diff oil.
 
If you decide to do the change over you will definitely notice the difference.