Photos may help. Determining whether it's an abscess that can be drained would be a good idea. If there is pus, then getting it cleaned out will help. If it's a tumor, then it would be best to leave it be. Also knowing where it's located can help as well - could it be an impacted oil gland?
I agree - dosing can be all over the place for injectable Tylan50. It has a wide range and can be frustrating to know how to figure out your dosage.
Page914 of the Plumbs Veterinary Drug Handbook ----Tylan50 dosage at 10-40mg per Kg 2-3 times a day.
Finding that "happy medium" I would go with 20mg per kg 2times per day. Technically for your 4lb. hen it would be .7272727272cc or ml, but since Tylan50 has such a large dosage range and seems to be well tolerated, rounding up to .80 for ease of measurement would be o.k.
Here's a thread that has a link to the handbook - you can download it yourself:
https://www.backyardchickens.com/th...ndbook-revision-6-free.1189785/#post-18800068
fwiw - Penicillin can be found at most feed store like
Tractor Supply in the refrigerated section. Injectables are still avialable, so if she doesn't improve with the Tylan50, then that may be your next step.
Since she is weak - provide her with some poultry vitamins and extra protein like egg, tuna or meat in addition to her normal feed. Marek's is a concern, and it can cause tumors on the body, but I would think you would find more than just one. It could be from a wound that just festered.
Keep us posted and we look forward to your photos - load us up with them if you can - different angles and a couple that shows the whole hen so we can see where it's located too.