You don’t need the “Monster as Player Characters” section, it’s just there to help in a few of the cases the authors thought would be particularly likely. You can get everything you need from the monster’s statblock.
First, the LA +4 is the correct level adjustment, and yes, it means it is legal to play. The RHD you take are the RHD listed in the statblock: 5d8 (d8 because the ettercap is an aberration). So a 1st-level ettercap would have effective character level 10th (5 RHD, +4 LA, and 1 for their first class level). You cannot play an ettercap in a game whose characters are lower level than 10th unless you use the Savage Species progression.
For ability scores, we look at the scores the ettercap has and subtract 10 (if the score is even) or 11 (if the score is odd) to get the ability score adjustments. For an ettercap, that’s +4 Str, +6 Dex, +2 Con, −4 Int, +4 Wis, −2 Cha. All in all, not stunning.
Since it has 5 HD, the ettercap has two feats (one from 1st, the other from 3rd), which the statblock lists as Great Fortitude and Multiattack—those do not need to be the feats you take for 1st and 3rd, they’re just the “usual” feats for an ettercap. What I’m really pointing out is that neither is marked B, and the number of them matches what we would expect—the ettercap has no bonus feats. Bonus feats would be locked, unlike the usual feats.
Also, for the record, the “ettercap class skills” for those 5 RHD are going to be Climb, Craft, Hide, Listen, and Spot—the skills mentioned in the statblock.
And otherwise, you just get all the things listed in the statblock: climb speed, low-light vision, poison, and web.
But if you wish to actually play an ettercap, you may want to discuss that with the DM: the ettercap makes for a really poor 10th-level character, and that’s a headache for the DM. I wouldn’t allow it on those grounds alone. Instead, I would probably try to work something out to justify the ettercap as a 1st-level character with no level adjustment or racial hit dice, so it can just take the appropriate class levels the player actually wants, and stay inside the usual parameters for players. This would also be very much to your benefit, because those nine class levels you get out of it would almost-certainly be worth way, way more than what the ettercap gets.