I was thinking of trying my first all-grain mash with a recipe for a Weizenbier. It calls for a single decoction. Is this possible with a BIAB method? Does it matter or can I just ignore it? This will be my 4th brew. My 3rd was bottled 2 days ago, ~4.3 gallons of an American IPA made with LME. I might look for a Hefeweizen recipe without a concoction. I will probably do 3 gallons because I only have a 4 gal pot.
Single Decoction for BIAB
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You can't do a decoction with a single vessel. At a minimum you need two vessels: one for mashing and one for boiling a portion of the mash. The vessel for boiling the decoction can be significantly smaller than the mash vessel.
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Decoction with BIAB is doable, as long as you have another pot and a way to heat it. The hardest issue is scooping out grains since you have more water than in a normal mash with BIAB. But to be honest, your first all grain...I highly suggest skipping decoction and just do a single infusion or a step mash (heat to first step, hold for set time, then heat to next step, etc). I've been doing all grain BIAB for 12+ years and have only done decoction twice. First all grain, you just need to get the process down, hit your mash temps, volumes, etc...so IMO don't try decoction until you have some more all grain batches under your belt.
A general rule for BIAB is to use a kettle that is twice the size of your target batch. A little bigger is even better. There are ways to make smaller kettles work, but 3 gallons into a fermenter in a 4 gallon kettle will be a bit tough. You would need to add in a sparge, and then start your boil with your kettle near the rim (or take the efficiency hit of diluting a higher OG wort).
2 gallons in a 4 gallon kettle is more straight forward. This is the size that my girlfriend brews with her 4 gallon kettle. I brew a mix of 2.5 gal batches in a 5 gallon kettle and 5.5 gallon batches in a 10 gallon kettle.
I agree with the advice to skip the Decoction. Some say it makes a difference, but in a flavorful beer like a Hefeweizen, the subtle impact would be hard to pick out. Some say that adding Melanoidin Malt at around 2% of the grist will add some of the character from the decoction...that was my plan for a Dunkleweizen I planned to brew about a year ago but never got around to it. But skipping both the decoction and melanoidin malt is fine.
get a bigger pot...
3x your batch size is what you should probably consider. That will be enough to make high gravity beers (very large grain bills) and thus be able to handle anything smaller.
If you know you will never make a RIS or other high ABV beers...then you could get away with 2x batch size for your kettle. But the cost difference is not much IMO.
15 gal will cover ANY 5 gal recipe. Pretty much 5 gal is the standard batch size.
Thank you all for responding. Now I am sure that a concoction is out of the question for this round. Yes, I need a bigger pot and for that I need a more powerful burner. I also need a bigger fermentor etc. Little by little. I want to keep trying new things, but just 1 at a time. If my goal this time is my first all-grain with BIAB then I will keep other variables limited. Unless I score a pot soon I will take the advice of 2 gal batch in my 4 gal pot . Appreciate the feedback everyone. 
Read the Zymurgy from May/June 2019 there is a section about Decoction (pages 46-49) and they describe Earl'sches method. I've used this method a few times for my Oktoberfest and it definitely works and makes enough of a difference to justify the extra time/effort
I really should of checked back here before I brewed today. I tried a 4 gal BIAB in a 4 gal pot. Sure enough OG was super low.
madscientist451
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Low OG in BIAB is likely caused by improper grain crush, not doing a sparge, recipe problems or mash temp issues.
You can get a second 4 gallon pot for about $20 at Walmart. Mash in pot #1, pull the bag and do a dunk sparge in pot #2. Boil both pots and combine after about 1/2 hour when some of the water has evaporated. I've been using this method for a few years now and I my target gravity almost every time. For a 3 gallon batch, I usually have about 2.5 gallons in each pot, and leave about 2 quarts kettle trub behind when I dump to the fermenter.
I am not sure what they have in store, but I see this decent looking 5 gal pot for only $25 (with a 4 gal version for $23): Mainstays 20 Quart Stock Pot - Walmart.com
I have a few of these pots in 2-gal and 3-gal size (they are very thin but seem fine on my gas stove, made in India): Mainstays Stainless Steel 12 Quart Stockpot with Lid - Walmart.com
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