So, I've been working on converting Paizo's 3.5e Adventure Path 'Curse of the Crimson Throne' (some spoilers below) to 4e D&D, and I've come up with some general principles/guidelines: The most general point is that threat level should look similar - an EL 3 encounter in 3e is expected to be roughly as challenging as a Level 3 encounter in 4e. I'm also finding it works best to take it that the 4e PCs be the same level as in 3e - ie don't make my 2009 mistake of assuming 1st level 4e PCs are functionally like 4th level 3e PCs.
In general with the AP conversion I'm finding that 4e monster statting rules work very well for this 'story' based play mode, whereas IME they often struggle in status-quo sandbox mode. I find 3e the opposite btw!
I'm generally finding that:
1) Once the PCs have a few levels, most NPCs look to convert best to 4e at '3e level+4'; for monsters it looks more like 'hit dice+4'. At very low level it's more like
PC Level - NPC 4e Level
1 3e Level+1
2 3e Level+2
3 3e Level+3
But this will need tweaking for plot importance etc. In general it's easy to threaten low level 4e PCs and a simple 1:1 level translation will often work, but after ca 3rd level this will make the NPCs too weak to threaten. Usually the NPCs' level should work out to be within 3-4 of the PCs' level at the time of the combat encounter.
2) If there are more than half a dozen creatures in an encounter then some convert best as minions. If they are very low level then they need extra levels, up to +8; keep an eye on the 3e EL to get a similar threat level overall.
3) There are a few obvious 'name' characters like Sabina Merrin in CoCTC that convert best as Elites at '3e level = 4e level'. Elite status comes with a huge wad of hp and is best reserved for NPCs with some 'plot protection' - it ensures they won't die fast which is good if you want/need to keep them alive, but risks tediousness if overused.
4) Solos - in principle a lone 3e creature of CR 5 makes a 4e level 5 Solo. These kinds of encounters should be rare though; often better to use an elite + lower level standard or minion allies to fill out the XP budget.
5) Statting needs to be thematic, not literal. Decide
(a) 4e Role & Level - this gives you the base 4e stats
(b) what is the interesting thing about this person/monster: base its powers around that.
In general with the AP conversion I'm finding that 4e monster statting rules work very well for this 'story' based play mode, whereas IME they often struggle in status-quo sandbox mode. I find 3e the opposite btw!

I'm generally finding that:
1) Once the PCs have a few levels, most NPCs look to convert best to 4e at '3e level+4'; for monsters it looks more like 'hit dice+4'. At very low level it's more like
PC Level - NPC 4e Level
1 3e Level+1
2 3e Level+2
3 3e Level+3
But this will need tweaking for plot importance etc. In general it's easy to threaten low level 4e PCs and a simple 1:1 level translation will often work, but after ca 3rd level this will make the NPCs too weak to threaten. Usually the NPCs' level should work out to be within 3-4 of the PCs' level at the time of the combat encounter.
2) If there are more than half a dozen creatures in an encounter then some convert best as minions. If they are very low level then they need extra levels, up to +8; keep an eye on the 3e EL to get a similar threat level overall.
3) There are a few obvious 'name' characters like Sabina Merrin in CoCTC that convert best as Elites at '3e level = 4e level'. Elite status comes with a huge wad of hp and is best reserved for NPCs with some 'plot protection' - it ensures they won't die fast which is good if you want/need to keep them alive, but risks tediousness if overused.
4) Solos - in principle a lone 3e creature of CR 5 makes a 4e level 5 Solo. These kinds of encounters should be rare though; often better to use an elite + lower level standard or minion allies to fill out the XP budget.
5) Statting needs to be thematic, not literal. Decide
(a) 4e Role & Level - this gives you the base 4e stats
(b) what is the interesting thing about this person/monster: base its powers around that.
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