What if I ran Expedition to Greyhawk (lvl 8 adventure) with 8 4th level 5e characters

[Ozreth]

So, when my players and I stopped gaming a few weeks ago they were level 4. I'm looking to run a mega adventure with them soon and would love to use Expedition to the Ruins of Greyhawk. It's for four 8th level 3.5 characters, but what if it were 8 4th level 5e characters? What are the major problems I would encounter here? The obvious is that when the PCs get hit it will be for a lot of damage, and it will be harder for the PCs to hit monsters (although they will have twice as many chances to do so). What else? Is this insane?

[Arzanyos]

The numbers are way different. A Stone Giant in 3.5, an appropriate challenge for a level 8 party, has the same AC as the 5e tarrasque, and can hit a 5e PC bedecked in Full plate and a shield, on a 3. that's kind of a problem.

[Jamesps]

So, when my players and I stopped gaming a few weeks ago they were level 4. I'm looking to run a mega adventure with them soon and would love to use Expedition to the Ruins of Greyhawk. It's for four 8th level 3.5 characters, but what if it were 8 4th level 5e characters? What are the major problems I would encounter here? The obvious is that when the PCs get hit it will be for a lot of damage, and it will be harder for the PCs to hit monsters (although they will have twice as many chances to do so). What else? Is this insane?

You could probably pull it off with a bit of effort. First off though you'll need to go through the encounters and replace all of them with their 5th ed versions. This will avoid problems like the above poster mentioned.

Also; I'd strongly suggest letting everyone level to 5th before you start. 5th ed works on a tier system, and there is a sizeable jump in power between 4th and 5th level. 5th level characters are on the same 'tier' as 8th level encounters, which makes them at least feasible if very difficult.

[Ozreth]

You could probably pull it off with a bit of effort. First off though you'll need to go through the encounters and replace all of them with their 5th ed versions. This will avoid problems like the above poster mentioned.

Also; I'd strongly suggest letting everyone level to 5th before you start. 5th ed works on a tier system, and there is a sizeable jump in power between 4th and 5th level. 5th level characters are on the same 'tier' as 8th level encounters, which makes them at least feasible if very difficult.

Yeah, I prep my encounters beforehand for the most part when running modules so I would be converting before we play, I'm just worried about whether things are close enough equivalents, but as the other poster noted I don't think they will be.