How can I change a solo monster to fit a party of 4 players? I simply reduce HP proportionally or do I need to make more changes? Can I do the same for elite monsters?

May I ask for a little clarity on this issue? Are you intending to use one of the solo monsters from the Monster Manual? If that's the case you probably want to slash it's hit points by 1/2 regardless of party size - this is particularly true for lower level parties such as yours. Solo monsters in general, but particularly from MM1 & 2, suffer greatly from HP bloat which just drags them out into long boring matches where players and the monster trade at-will attacks until one of them eventually falls over (see Hydras for the ur-example of the awful design).

As for some actually useful advice;

• Cut hit points by at least 1/3rd but as mentioned above feel free to go as low as 1/2.
• Reduce it's highest defense by -2. It's going to be attacked less then it would normally be so doesn't need the innate bonus to defenses all solo creatures get.
• If the creature has multiple ways to attack each turn (and it should) impose a limit on how many of them can be stacked against a single target each turn. Generally 2 attacks per person is a good rule of thumb to go by (although attacks which do not inflict damage are usually exempt from this rule).

That's about it for general advice. As for elite creatures I'm not entirely sure what your asking here. Do you want to buff up an elite creature to take on a group of four players alone? Or are you asking if you need to make any changes to a elite creature for a group of four? If it's the latter then the answer is no - an elite creature is the equivalent of two creatures in battle as as such no changes should have to be made (unless your planning to throw three of them at the party at once or something).

Oh yeah. If it's a creature you've built yourself I'd have to see its stats to offer real advice on that. In general a little more lethality in exchange for a lot more fragility is always the way to go.

The solo monster was custom made using the DM guide 1, but it was made to fight 5 players. With only 4 players, they will have a real hard time fighting it. I will send a PM for you with the monster status.

I have an encounter with 2 elite monsters and other creature, but I don't want to remove any of the fight. So I was thinking in removing HP from the elite monsters to make them a little easier.

Okay fair enough. In the case of the elite creature scenario, assuming it supposed to be a hard fight, your probably good as is. Nothing wrong with occasionally throwing a fight a little stronger then normal at the party to keep them on their toes. If your worried about the elites bringing too much to the table and upsetting the combat scenario you could consider cutting them down to 1/2 hit points, effectively making them normal creatures that just happen to have two attacks per turn and slightly better defenses. If the party is worth it's salt they should single out and attack those two anyway.

I am back here just to say my public thanks to Garren. He helped me a lot with my solo monster.

From what I've seen, Garren does in fact know his stuff quite well.