In a normal Pathfinder game, all characters in a party are of equal level, or at least of roughly equal. In PFS, however, it can range from 1 to 5 in a single module that I will attend, and it feels rather weird.
How can one contribute to the party if it happens that the other 4 players are all of higher levels? How not to be a burden for the party?
The reality is, you don’t. Pathfinder is a game where characters, at least some of them, are doubling or more in power every couple of levels. Being a level or two behind is massive. Being four or five behind basically means you could be playing different games entirely. Especially when your level is 1st—even a 2nd-level character is playing a fundamentally different game from a 1st-level character. That’s just reality in the Pathfinder system—a reality that, unfortunately, neither Paizo nor the Pathfinder Society recognizes.
On the other hand, Paizo’s published modules as well as the Pathfinder Society also mostly cater to nearly the lowest common denominator as far as optimization is concerned. If you choose a high-tier class, and optimize it well, you might be able to overcome these limitations somewhat, as long as you are playing with people who aren’t doing the same. Even if they are, you might at least be able to interface with the challenges of the games you play, even if those around you are blowing through them with ease. Still, this is likely to be a frustrating experience, it might draw the ire of GMs, and it might turn into you being the cause of problems after you’ve leveled up some and now have lower-level characters in your own party. It’s not a great solution.
But you can somewhat improve this situation by focusing on support. Support means you don’t have to interact with saving throws, AC, SR, or whatever other defenses the challenges may have—which is very good because your numbers will be difficult to get to a point where that will be anything but a waste of time. You don’t need any special boosts to provide inspire courage, for example, to your party. It won’t be much but it will add to what the party is doing. When the forces you’re alongside are devastating, being a force multiplier is effective. And it also means that when and if you level up and end up in the opposite situation, you’ll be able to support your lower-level allies and help them to survive where they aren’t supposed to be (or, at least, avoid overshadowing them as they deal with things where you aren’t supposed to be).
Semada’s answer is also good—not getting in the way is a bare minimum here to avoid a really frustrating experience for all involved. If you manage to do that much, then the game might be a frustrating experience only for you. Which isn’t much of a win, but it might be the best you can get in this situation.
How not to be a burden for the party?
If you are unfortunate to be in a party of characters significantly higher than you are, the best way to not be a burden is to not become a liability. Don't get hurt so the cleric has to spend resources healing you. Don't get in the way of the charging barbarian. Don't inadvertently provide soft cover to an enemy against your party archer. Really, all the same things apply if you are equal level. Being at a lower level just amplifies the likelihood of them happening.
I dont know what class you are playing, and that's an important factor in this question as different classes are more effective at certain levels than others. A low level cleric can be extremely useful to a higher level group if they don't have any source of healing aside from you. On the flip side, a low level fighter might not be more useful than as a flanking aid in a higher level group.
Mentally remove yourself from the group and see where they fall short in terms of capabilities, then try and find ways your character can support them in those areas. Be a good role-player as well. Come up with creative ideas outside of combat.
The higher level players should understand that its PFS and having lower level players in the group happens and to not make you feel bad about it. Good players will try and help make you more effective and keep you involved. The bad ones will tell you to just accept being powered through the scenario.
Talk with your party (as much as you can in PFS) before hand and see if they have any ideas where you can assist and fit in.
The real answer is that you shouldn't join the higher tier scenario with the lower tier character.
PFS scenarios split each tier into two subtiers. For example, a 1-5 scenario can be played either low (1-2) or high (4-5), having different encounters, save DCs etc. Playing out of tier is never fun for player, party or GM. Level 4 character will obliterate lower tier challenge, and high tier encounter will probably be deadly for the lower level character.
If you don't have a character of the appropriate level for the adventure, grab a pregen instead and apply it to your character once it reaches the requisite level.
