I just beaten Ultima 4:

http://www.gog.com/forum/general/i_have_beaten_ultima_iv/page1

So now I am going to play Ultima 5, so I wanna know any tips, recommendations, and cautions about the game.

And any differences between Ultima 4 and 5 I should know?

There are quite a few differences: better graphics, improved combat controls, an actual inventory, improved spellcasting, you can buy ships, there's a karma system instead of the partial avatarhood of U4 (since you are the Avatar) and there's an actual day/night cycle with NPC schedules.

As for tips, since Lord British went missing he won't be able to freely heal you and to level up you need to sleep, although I don't think the level up always triggers.
If you enter a town and you get a message reading "An air of x surrounds you", GET OUT and come back in a couple of days.
The difference between the 3 classes your party members can be is their potential spell points and the starting attributes: as such mages tend to be the best party members since, eventually, they will be able to wear the best armor and wield the best weapons while also being able to cast powerful spells.
Dungeons are now locked, so you will need to find the member of the Council in hiding and ask them for the words of power to open them, but be careful because while dungeons have a fair amount of treasures, they are also a lot harder to get through.

There is also a runeless mod for Ultima 5 that replaces all the runes with normal latin characters
http://nodling.nullneuron.net/ultima/ultima.html

i heartily approve of relying on mandrake or nightshade because while i'm not the most patient player, i have no problem with free harvests (i'm ashamed it took me so long to figure out that casting View was far more practical than buying Gems, at least in IV)...

but for practical purposes, is In Xen Mani actually cheaper than the cheapest ration packs in V - even if all of your mandrake is manually harvested? i'd always thought of it as a curiosity more than a utility (especially being taught by a jolly, indulgent gourmand), but maybe that's because i've ended up relying less on a diverse spell repertoire in V than i ever do in IV - despite massively preferring Mages and hating the betrayal of the versatility of Paladins and Rangers...

well this should be embarrassing for all of us...

check out the first item on this page: [url=http://strategywiki.org/wiki/Ultima_V:_Warriors_of_Destiny/Side_quests_and_miscellaneous_items]http://strategywiki.org/wiki/Ultima_V:_Warriors_of_Destiny/Side_quests_and_miscellaneous_items[/url]

Devote some early time to gaining a level and the stat that comes with it, then purposely dying. Why? Your EXP will drop, but your stat gain remains. Thus, you can bounce back and forth from say, Level 2 to 3, and gain a stat point each time. This makes the final dungeon doable, especially if you get high Dext. You'll be able to take 2 turns to a dragon's one turn, sometimes more.

To sort of cheat for an experience gain, find a dungeon room with some tough monsters, and have most of the party flee while one player remains in the room while wearing an invisibility ring. This not only hides the player but puts the monsters in flee mode....one shot kills. But DO NOT clear the room. Why not? Because that room will lose its experience farming. Leave one monster alive, then come back in and they'll have respawned. There is a really good room in Level 7 of Shame that has 4 dragons and even better treasure. Like loads of it (magic shields, magic axes, potions, etc). Do the same thing as above. Kill 3 of them, leave, and come right back. 75 EXP a pop, and a ton of treasure each time.

Also, that final dungeon, hard as it can be, is easy if you mix up a bunch of "cause fear" (?) spells. That puts the monsters in flee mode, making them super easy to kill. One hit usually does it.