Peppers, especially some of the hot ones, can take ages.
I'd say after 4 weeks they should be up though.
I struggle to maintain even heat and water for the time it takes peppers to germinate.
Sounds like your brassicas have damping-off, a fungal disease. I'd consider them done-for.
I'd start them again in FRESH seed-raising mix and make sure the surface is dry before you water the seedlings.
Damping-off thrives in wet, humid conditions.

If your planting media wasn't sterile, there are a number of soil born fungi and whatnot that can take out your seedlings pretty easy - and the typical symptoms sound like what you're describing.

As for your peppers, I don't know if it's required for peppers, but I do it for all of my seeds by habit - some seeds require a cold period before they'll germinate well - storing them in a cold refrigerator will normally accomplish this.

Also, temperature is very important with peppers. It's not enough to toss in a heating pad and call it good - have you measured the soil temperature? They prefer it around 80-85 typically if you can manage it. Lower temps just mean they take longer to germinate and have lower overall germination rates. Normally it can be managed if your temps are at least 70, but it may take a month.

More typical times for peppers are 1-2 weeks if your temps are decent. Mine are hovering around 74 degrees and I just got my first two coming up today - 7 days after planting.

You also have to make sure the seeds stay moist.

Where did you get the seeds, and are they all one variety or several? I've heard that hot peppers germination may take longer that sweet peppers on the average and I grow almost solely sweet, so my experience with hot peppers is limited.

4 weeks is a long time even for the slowest pepper seeds. Do you have a dome over your seed tray? I find that even with the heat mat, it is much faster and easier to germinate peppers (and eggplants) if a dome is able to keep the moisture contained. I will remove the dome once seeds start to pop up. This is far better than keep watering the seeds.

Peppers don't need a cold period before planting. Some varieties can take a lot longer to germinate than others. What type are you growing? As mentioned, you need to know what the temperature of your soil is. If you're using a regular heating pad, it may be too hot and the seeds will cook. What kind of set-up are you using? Is it a tray with a dome cover like a germination kit? Are your brassicas covered? A bit more information will help us help you.

On the watering bit.. I'll share this since a lot of people don't seem to be aware of it and I've been doing it for years with great success.

I take the 18 ounce plastic drinking cups (keeps me from potting up later) and poke several holes along the bottom rim with a push pin (stick pin). I then fill them with my growing media and place them in a shallow tray filled with water (currently using plastic rubbermaid totes from WalMart).

This makes watering easy - the soil will wick the moisture up to the top of the cup and always seems to be the perfect amount of water - not too much or too little. All I have to do to water is keep the pan full, which after they're going (they'll suck up a lot of water initially) means I water once or twice a week. Depending on your media, it might be necessary to wet the soil yourself the first time to get the wicking started. If you leave it to do it by itself, expect 2-3 days before the soil is moist all the way to the top.

This not only makes watering REALLY easy, it also insures steady moisture for your plants instead of the fluctuation often seen with typical watering. I've used this with great success for years and love it.

My peppers and other seedlings (broccoli, cabbage, onion, leek, tomato) don't have a dome over them, just the heating pad underneath. I let the soil dry a bit before watering and the temp is about 75 degrees- I'm growing sweet and hot peppers. Chinese Giant, Fish, Cheyenne hybrid, Habanero (Chocolate, Orange, and White), Jimmy Nardello, Corno Di Toro Rossi, Fatali, Holy Mole, Hot Portugal, Sweet Chocolate, and Sweet Banana are the ones I've started. Only 1 Fish has sprouted. Some of these seeds were from trades I did a few years ago so I could understand why those would take longer, but the majority were saved seed from last year, purchased or traded. The traded ones were from last year and were fresh from the previous years growing season.
The cabbage and broccoli were started in Jiffy pellets and the other seeds were in a compost/peat mixture. The tomatoes are doing awesome, though I've had to snip 3 more seedlings that wilted. It's a little cool and windy right now, but plenty warm enough to put them outside for awhile.

Peppers definitely don't need cold period before planting. For the brassicas, damping off is probably what you have. You should dump all the soil out and clean your trays with bleach solution (9 part water, 1 part bleach) and get new starting mix.

There's a trick to quick germination of pepper seeds. Wet the media then let it dry out completely (bone dry) then rehydrate and keep it moist. I usually get 80% germination in 3-5 days after re-hydration. Not sure I would try it at this point.

I agree that the problem with others is probably damping off. Try not keeping them too damp or using a fan. Any media that has had this happen in should be dumped and containers cleaned as mentioned above.

Chinense, like habs, can be really slow to germinate.
I slip my trays into plastic bags to maintain even-ish moisture.
I'd start a new batch alongside, otherwise you might miss out altogether!

75 degrees may be a little too cool for pepper seeds...they like it about 85 degrees. Still, you should have had more germination by now. I agree with Feijoas that you should start some back-ups, and keep the medium a bit warmer.

In my experience (and I grow mostly sweet peppers, but I've tried a ton of different varieties (I've got 17 on my list just for this year)), you can do it at 75. Warmer is better, but it can be done at 75. This year I'm struggling with my temperature situation and I'm only managing around 74F. 7 days after planting my first 2 popped up. The next day I had 7, I haven't checked yet today. I planted around 70 seeds total. I expect the bulk of them will have come up by 14 days after planting. In the past with cool germinating temps, I've had some take a month to pop up. Normally if they take that long though, they're not coming up at all.