The young simple template would take it down by 1. I'd skip the size reduction though, as going from small to tiny is a big deal.
Alternately, you could just take the advanced simple template and apply it backwards (subtracting instead of adding) each time should reduce the CR by 1, although I would probably at most adjust the AC once, or even not at all as 15 isn't all that high.
So with 2 advanced reductions not counting AC we would get
HP: 26, 2 slams +3 1d6-1 damage each, grab and paralysis. CMB of +1 (+5 grapple) paralysis DC of 14. AC Still 15.
Seems about right for a CR2 to me.
^^ Dave has the right idea here. The damage ends up being low for CR 2 average, but it works out fairly well. It fits the Monster Creation guidelines in the Bestiary for the most part.
First things first: bookmark this page, either in your browser or in your physical copy of the bestiary. It's your best friend for monster creation/modification, and tells you what kind of power level you're looking for in a specific CR range.
Now, let's look at the Slithering Tracker and see how it compares to the benchmarks for a CR 4 creature on the first table.
HP: 42 vs 40 <= bang on
AC: 15 vs 17 <= slightly low
Attack: +7 vs 6-8 <= exactly in the middle
Damage: 13 vs 12-16 <= closer to the low end of the benchmarks
Ability DC: 18 vs 10-15 <= well above benchmarks
Fortitude Save: 7 vs 3-7 <= on the high benchmark
Reflex Save: 5 vs 3-7 <= middle-ground
Will Save: 1 vs 3-7 <= well below benchmark
So now that we know where it stands in respect to CR 4 benchmarks, we can simply map that to the CR 2 benchmarks to get our target stats.
HP: we want about 20 hit points
AC: We want to be a little low, so 12 AC
Attack: middle-ground, so 3-4
Damage: closer to the low benchmark, so about 7
Ability DC: above benchmark, so 15-16
Fortitude: high benchmark, so about 5
Reflex: mid-range, so about 3
Will: below benchmarks, so 0
Now that we have target numbers, we can begin to rebuild it. The first thing we should do is reduce its hit dice. Dropping it from 4 to 2 hit dice should bring down a lot of its stats to the range we're looking for. Hit points especially should be right on the mark after this. Other stats will need further tweaking. To help bring it into line, we can drop some of its stats. Just eye-balling it, I'd say that reducing the strength by 2, dexterity by 4, and constitution by 2 points each should come pretty close, but also reducing the damage dice one step would be advisable.
After applying these changes, I recalculate the various stats of the creature to produce its new statblock:
Slithering Tracker - CR 2
xp 600
N Small Ooze
Init +2; Senses blindsense 60 ft; Perception +2
AC 13, touch 13, flat-footed 11 (+2 dex, +1 size)
HP 19 (2d8+12)
Fort +5, Ref +2, Will 0
Immune mind-affecting effects, ooze traits
Speed 10 ft, climb 10 ft
Melee 2 slams +4 (1d4+2 plus grab and paralysis)
Special Attacks blood rain (1d2 constitution), grab (colossal)
Str 14, Dex 14, Con 21, Int 11, Wis 10, Cha 1
Base Atk +1; CMB +2 (+6 grapple); CMD 14 (can't be tripped)
Feats Skill Focus (Stealth)
Skills Climb +8, Perception +2, Stealth +16; Racial Modifiers +8 stealth
Languages undercommon (can't speak)
SQ Transparent
Paralysis - DC 16
This is pretty close to the numbers that were ballparked earlier for a CR 2 version of the monster. The point isn't to be exact, so pretty close is good enough. It's still a little high for damage, so you could knock some more off of strength if you're concerned, but it's still inside the overall CR 2 benchmarks so I think these numbers are fine. Often times you do want to remove or weaken some abilities when reducing a monster's CR, but going from CR 4 to CR 2 isn't a big enough jump to necessarily merit that.
The last step is the sanity check: look over the statblock and ask yourself "does this look like a CR 2 monster?". To me, the answer is a yes, so I'd say this is ready to run.
Hopefully you could follow the process so you'll be able to do it yourself in future now :-)
All that is good and all, but...
Personally I don't see anything wrong with having a party of 4 2nd-level adventurers face off against a CR4 slithering tracker. It might be somewhat challenging, depending on the circumstances and the level of optimisation of the PCs, but I bet that if it's a solo creature it's not going to last more than 2 or 3 combat rounds.
If you play the environment and the critter's stealth and hit & run tactics to the hilt, you might get more mileage out of the encounter, and make it a dangerous and memorable foe.
But if you slice the poor slithering schmuck down to CR2, you'd better give him some pals, say 3 or 4 or more of those beasties, or else it's going to be a quickly-forgotten footnote in their adventuring day.
Fast & easy: replace Paralysis with Sickened, and leave everything else the same.
One thing I find. Don't get overly hung up on the CR. Oftentimes having the monster fight sub-optimally (not dumb just not maximized) often leads to fun fights.