Firstly have a look at Ralph's excellent
MaxiBIAB Guide
As in the guide he usually uses 4.5kg of grain for his standard brew length and up to 5.5kg can be mashed in a 19L pot using this method.
However if you're set on using only 4.5kg of grain you could add a couple hundred grams of DME to the boil to get your gravity. For an LCPA type beer you want a starting gravity of about 1053.
So try this recipe;
Batch Size: 20.00 L
Estimated OG:
1.053 SG
Estimated IBU: 42.0 IBU
Brewhouse Efficiency: 75.00 %
Boil Time: 60 Minutes
Ingredients:
------------
Amount Item Type % or IBU
0.25 kg Light Dry Extract (8.0 SRM) Dry Extract 5.26 % (15min)
2.80 kg Ale Malt (Barrett Burston) (3.0 SRM) Grain 58.95 %
1.10 kg Munich I Malt (Weyermann) (7.4 SRM) Grain 23.16 %
0.30 kg Carapils Malt (Weyermann) (2.0 SRM) Grain 6.32 %
0.30 kg Wheat Malt Pale (Weyermann) (2.0 SRM) Grain 6.32 %
5.00 gm Chinook [11.40 %] (60 min) Hops 6.8 IBU
15.00 gm East Kent Goldings [4.80 %] (60 min) Hops 8.6 IBU
12.00 gm Cascade [5.00 %] (20 min) Hops 3.9 IBU
12.00 gm Chinook [11.40 %] (20 min) Hops 9.9 IBU
12.00 gm Chinook [11.40 %] (10 min) Hops 5.9 IBU
12.00 gm Cascade [5.00 %] (10 min) Hops 2.3 IBU
12.00 gm Chinook [11.40 %] (5 min) Hops 3.3 IBU
12.00 gm Cascade [5.00 %] (5 min) Hops 1.3 IBU
20.00 gm Cascade [5.00 %] (Dry Hop 5 days) Hops -
20.00 gm Chinook [11.40 %] (Dry Hop 5 days) Hops -
1 Pkgs American Ale (Wyeast Labs #1056) Yeast-Ale
But if you want to keep it strictly all grain you've got 3 options;
1. try and fit more grain into the pot to get to your OG
2. make a smaller brew length to the same OG = less beer
3. lower your expected OG based on the amount of grain that you can comfortably mash. Since you've already said you're comfortable only getting 4.5kg in the pot. Then try this;
Batch Size: 20.00 L
Estimated OG:
1.048 SG
Estimated IBU: 38.9 IBU
Boil Time: 60 Minutes
Ingredients:
------------
Amount Item Type % or IBU
2.80 kg Ale Malt (Barrett Burston) (3.0 SRM) Grain 62.22 %
1.10 kg Munich I Malt (Weyermann) (7.4 SRM) Grain 24.44 %
0.30 kg Carapils Malt (Weyermann) (2.0 SRM) Grain 6.67 %
0.30 kg Wheat Malt Pale (Weyermann) (2.0 SRM) Grain 6.67 %
15.00 gm East Kent Goldings [4.80 %] (60 min) Hops 8.9 IBU
20.00 gm Cascade [5.00 %] (Dry Hop 5 days) Hops -
10.00 gm Cascade [5.00 %] (20 min) Hops 3.4 IBU
10.00 gm Chinook [11.40 %] (20 min) Hops 8.5 IBU
10.00 gm Chinook [11.40 %] (10 min) Hops 5.1 IBU
10.00 gm Cascade [5.00 %] (10 min) Hops 2.0 IBU
10.00 gm Cascade [5.00 %] (5 min) Hops 1.1 IBU
10.00 gm Chinook [11.40 %] (5 min) Hops 2.8 IBU
5.00 gm Chinook [11.40 %] (60 min) Hops 7.1 IBU
20.00 gm Chinook [11.40 %] (Dry Hop 5 days) Hops -
1 Pkgs American Ale (Wyeast Labs #1056) Yeast-Ale
Note that the IBU has been lowered also in the "lighter" version. This is to maintain the balance. Just reducing the grain amount and keeping at the same IBU will unbalanced overly bitter beer.
BTW... if it were me... i'd be trying to get as much grain into the pot as possible. You could always test it out by adding all your spec malt to the mash then progressively adding your base malt till you can add no more. Then adjust your hop addition based on the post boil gravity to create your target IBU. I reckon you could get more than 5kg in there.
Edit: for 20L brew length
