That
#4 Cu is worth 78 cents a foot or $780 the spool.
Given your requirements, the wire you want for the garage is
#4 Al at 26 cents a foot, which will give you *very* acceptable voltage drop, *even in Canada*. Normally I'll chintz out on wire and go 20 cents/foot #6 Al, but going to #4 removes the need to have 3 different colors of wire. So you can get it all done with one spool, and don't have to buy by-the-foot.
"But what are you talking about Harper? **I already own** the copper wire!" Yeah, sell it for darn good money. With the savings, buy the Al wire, and the copper for the garden branches, and most of the conduit lol.
"But aluminum bad!" Not for feeder of this size. It's just the right stuff.
#4 Cu is an awkward size anyway, too big for 60A too small for 100A.
On the garden outlets, absolutely not. You do not have the feeder "make a stop-off" to serve a receptacle. Feeder cannot feed receptacles. It *could* serve a subpanel there, but I don't think you want a subpanel in your garden
Simply throw a 1/2" PVC conduit in the trench, with three #12 Cu's (12 cents/foot) in the pipe (black white green) and you're done there. That can breaker for 20A, except in Canada you'll need a 15A nanny breaker to keep you from having, OMG, 3.7% voltage drop (like, literally, who cares?)
Don't even bother having the #12 and #4 share the pipe, because you can't just have an underground "Tee": you need 2 stub-ends to surface the box, an outdoor splice box, outdoor splices, etc. etc. And if you ever re-landscape and want to get rid of that box, you can't without replacing all the cable. Not worth it.
Once you get to the barn, you'll need to splice the #4 wire to get down to a size that will go on a receptacle. Those splices are
ex-pen-sive and may not allow you to do it one step. But hey,
here's one that provides 4 appropriate splices in one box, plus some other stuff you'll find useful.
That was easy!
